First of all, before I even open Google SketchUp, I gather together a directory of reference material. Particularly important is any schematics I can find, and in this I was very lucky - the plans I located on-line even included all the measurements!
One very nice reference picture I came across is a shot of someone else's 3D rendering of the BTR-70. It is very interesting to study this, as you can see what compromises they made in leaving out certain of the more intricate detail.
In this case the model is highly detailed, with a lot of surface ornamentation. I hope my model looks as nice as this when I'm finished!
First steps to a 3D model...
The very first thing I do inside SketchUp is to arrange my 'template' schematics. As this is a very quick and dirty model, I will be tracing my profiles directly off the templates (and then extruding the profiles to make the correct shapes). But before that I must scale the schematics to the correct size - this is where the measurements - conveniently included - come in very handy!
I centred the templates up on the main axis, but on two separate layers which I could turn on an off when I wanted to check the plans for reference. This is the beauty of SketchUP, it makes the process of acquiring your principal shapes from pictorial imports very easy.
Studying the overall shape of the vehicle, separated out the sub-component shapes in my mind. Obviously the largest cohesive shape is the crew cabin - everything else hangs off this, so it was a good place to start.
Looking at the profile plan and cross-section you can see that the main body of the BTR-70 is an odd shaped tube, made up of several flat panels at sloped angles. This makes it very easy for me to draw out the profile and then extrude the shape of the main cabin. No nasty organic sweeping curves here, comrade!
The above picture makes the first step in the modelling fairly clear - trace the profile, and then pull the cross-section into a long tube shape. Inset is a view down the tube - all other parts of the main body hang off this tube, it's now a simple matter to shape the 'nose' and the rear of the BTR-70.