Hi Brittair. It might be easier to convert an existing aircraft to use as a macro rather than create your own. M98toBGL posted in the SimV Scenery Design section can be used to convert most FS98 aircraft into API macros.
As I recall, the FSDS texture template feature will create an outline image of the selected part. This can be used as a guide for painting your finished texture. It's slightly oversized & there should be lines indicating the actual area covered by the texture depending on the resolution you selected. This can be cropped/resized to suit or pasted into another texture of the correct size. Several parts can be included on the same texture with a little careful planning.
PS. This took less than 5 minutes with M98toBGL & FSSC.

There are several versions of Massimo Taccoli's Tornado posted in the FS98 Military section.
PPS. Ooops. Slight problem with that macro. If you look closely you can see 2 sets of wings. This is caused by the older style animation of the variable geometry wings. It's possible a Tornado by a different author would be OK if you can find one.