*** I've reserved a few posts so the other tuts can be added later and will be above posted comments/questions ***
Tut 1 Making an outtershell or decorative building
Tut 2 Making an innershell
Tut 3 The 'Alternative' way
Tut 4 UV mapping
Tut 5 Lightmapping for UDK
Tut 6 Special shapes
TUTORIAL 1 Making an outter shell or decorative buildingThis tutorial is meant for beginners and people who never worked with Blender, so I'm modelling in the simplest way possible. Might not be the 100% correct way, but people have to start somewhere, don't they ? To download Blender, go to Blender.org and download the latest version, it's free.
Prepping in GTKradiant
Let's take this building on the SpecOps side of the Northport map for example, I chose this one because it has 2 different rims on the roof, making it perfect to demonstrate modelling in different ways. :::

Let's go into GTKRadiant and single that building out. Oops, what's that doing there ?

Let's clean it a bit first and then export it.
Select the building and export it with plugins->Brushexport2->Collapse mesh-> be sure not to forget to add .obj to the name


Now we're done in GTK, let's hop over to Blender.
Import obj and from the Object tools at the left side menu, select 'Geometry to Origin', which is the first option.

Then press S0.01 <enter> RX270 <enter>
This scales it to 1% and Rotates it 270 degrees on the X-axis.

If everything is alright you will have the following things in your scene : Camera, Cube, Lamp and all(which is our imported building model). In the following picture you'll see that the object ALL doesn't have an eye behind it, this means the object is hidden. We don't need it to be hidden for now. Let's begin by selecting the cube.
Some useful things you should know<ALT>+<left mouse> to pan
<ALT>+<shift>+<left mouse> to move around
<ALT>+<CTRL>+<left mouse>+<move mouse up/down> to zoom in or out
I have these settings because I have a trackball mouse, we'll go over Blender's settings in a seperate post if anyone is running into problems.
At the bottom of the screen you'll find some important icons we're gonna use a lot.
At the left side you'll see the edit mode, on the picture it's set to edit mode.
There's also an icon with a Red,green and blue icon, the 3D manipulator widget, use this if your manipulator widget is invisible.
Circled in red are
Vertice, edge and face select throughout the tutorial we'll be using those a lot and will be marked in
Blue
Now we've got that down, let's start
Grab the cube, place it in a corner of the building and go into Edit mode

Now we're going to select the left face, select the
Face select icon (right one)
To be sure we're selecting the right thing, first press
<A> to unselect, now your cube should be grey. If it's yellowish push
<A> again.
Now press
<C> for the round selector and click on the left side of the cube. If you selected more than that face, use
<ALT>+
<Leftmouse click> to deselect.

Press right mouse to get rid of the round selector (which can be sized with the
+ and
-) Then use the widget to move the face to the left side of the building. If your face is not really aligned, look which axis we're moving in (this case the Y axis) and press
Y on your keyboard. When moving a face or edge I tend to move the mouse up and down to see if it's locked on the axis I want to move it in. If it's not locked then we might get a crooked thingy like on this pic

This is how it's supposed to look

Press
<A> to deselect,
<C> to bring up the round selector and select the back face, then right click so we can work with the translation widget again. Now bring the face to the back.
Press
<A> to deselect,
<C> to bring up the round selector and select the top face, then right click to bring up the maipulation widget again. Now pull the face upwards. If you did this correct, it should look like this

Now, we're going to make the roof.
Go into the right top menu and hide our original building by clicking on the eye behind the ALL object.
Set your mouse on either the bottom or top and press
<CTRL>+
<R> and a purple line will appear.
Press left mouse button, turning the line to yellow, then press right mouse to move on.

In the case of this building, we're adding a line at each side with
<CTRL>+
<R> 
Bring back the original building with the scene menu at the top right.
Only click the eye icon, else it's going back into object mode.
Position your mouse at the side of the cube and then use
<CTRL>+
<R> to add a line
Click left mouse button and move the mouse to position the line at the same position as shown on the picture.
Now click left mouse button again to finalize.

Let's hide our original building with the scene menu at the top right.
Now click the
edge selector, which is the middle icon
Then press
<C> for the round selector and left click on the two top edges

Bring back the original building with the scene menu at the top right, only click the eye !
Now, with the two edges still selected, pull it up to under the roof.

Press
<A> to deselect
Select the
Face selector then
<C> to bring up our round selector.
Now select the four faces of the roof, then press
<E> to extrude and pull it out a bit

Now, let's move to the left side of the building.
Press
<A> to deselect
Edge select, middle icon
Press
<C> for the round selector and select the top edge

First pull it a bit to the left, then down

Go to the right side of the building
Press
<A> to deselect
Face select, right icon
Press
<C> for round selector
Select the face like shown on the picture

Press
<E> to extrude

Press
<A> to deselect
Press
<C> for round selector
Select the faces like shown on the picture
Press
<DEL> and select faces
Press
<A> to deselect
Press the
Edge select icon (Middle icon)
Press
<C> for round selector and select the two edges
Press
<F> to create a face

Do this for the back side too by repeating the last 2 steps.
Now we're gonna fill up the gap
Press
<A> to deselect
<C> to bring up the round selector
Select the edges like this on the front and back

Then press
<F> to create a face
Now that gap is filled, we're making the roof stick out a bit on the front(facade) of the building.
<A> to deselect
<C> for the round selector
now select the faces of the rim
Press
<E> to extrude
Now that's done, press
<A> to deselect and let's start making some windows.
Set your mouse at the left or right side of the facade of the building
Press
<CTRL>+
<R>, then Click left mouse button and move the line into position where the top of your windows wll come and press left mouse again to finalize. Repeat this for the bottom of your windows
<A> to deselect
Set your mouse at the top or bottom side of the facade
Press
<CTRL>+
<R>, then Click left mouse button and move the line into position, press left mouse again to finalize. Repeat this for all of your windows
<A> to deselect
<C> for the round selector tool
Left click on all the windows
Press
<E> to extrude and push it inwards a bit
<A> to deselect
Press
<CTRL>+
<R>, then Click left mouse button and move the line into position, click left mouse again to finalize.
Repeat this step for the other side of the door.
Note ::: If you are converting a map, be sure to position the lines
IN the middle of the metal door frame
With <CTRL>+<R> we add another line at the bottom to mark to bottom of the door/top of the door step.

We use a model for the door step, door frame and door itself(all is one model), so instead of this step where I extruded the door inwards and the door step outwards, we delete these faces.
Add two lines with
<CTRL>+
<R> for the rim and to seperate the top from the bottom, because they have different textures.
We don't need this floor, delete it.
<A> to deselect
<C> for the round selector
<DEL> to bring up the delete menu and select faces
The inside of our building, a neat little 'shell'. As you can see, the endresult is very different than modelling it in GTKRadiant. If the building is not meant as a prefab, delete everything the user won't see, in that case the whole back can be deleted.
Tutorial 2 will be about making an inner shell, so the building will be accessible to players.
BUT since you want to test your building asap and probably don't want to wait testing it on the Unreal Engine or any other engine of your choice,
I will post the chapter on how to UV map your building next. Note that this model is not nearly ready to be put into UDK yet in it's current state,
it will need a few more steps.
End of Tutorial 1
Have fun w/ Blender
Caesar77