[GRAH1]

Balsa wood is easy to carve ,obechi is good to but probably a little harder to find these days,
Trace the shape on the wood from the plan and cut the profile out with a band saw or a coping saw .If the plan is an old "model cars " plan there will be sectional templates included ,trace these on to card board and cut them out this will help get the cross section right .
hope this helps you get started

 

[GRAH1]

I ususaly use modeling clay when making a mold start with a 2.5" wide block car away ,when it goes a bit soft stick it in the fridge fore 1/2hr have a brew tak it out carve some more , then plaster cast and make a rsesin master to which you can engrave the final details