The instructions should have the pattern shown. If not hunt up
some photos of the piece you are modeling and use them. Notice
how patterns curve around panel lines, wheel wells, windows, etc. and
try to adjust them on the model so they curve the same way in the same
places.
Start with the lightest color and spray the whole model. Next
apply the medium color, and finally the darkest color. Always try
to go from lightest to darkest.
If you don't have an airbrush or don't want to use rattle cans you are
pretty much limited to hard-edged camo. Rattle cans can produce a
decent soft-edge camo if you are careful and use a soft mask to do
it. With an airbrush you can use a soft mask or spray it freehand.