ok, here's some pointers for you:
Take your time.
Follow all directions on the products.
More specifically related to your project:
- Straight lines are impossible by the hand, a jig or ruler is needed to guide the tool.
- Gaps should be filled in with the same material as what was cut. Example: ABS plastic should be filled back in with ABS. Other materials of similar properties can be used in place of the original if necessary, but BONDO is not a "fix-it-all." BONDO is designed to fill small dents and cracks to make a smooth surface, and should not be used for any structure.
- The best surface is achieved after a lot of sanding. I really mean a LOT. You'll also want to use a sanding block, or something other than your hand.
- There are a lot of techniques to try and match the texture. Some textures and patterns are not achievable with home-made tools, and a different texture will need to be used. There are spray on texture paints available in varying textures.
- Spray Paint (sometimes called Rattle-Can paint) can be used with great results. Hold the can a little further from the area that's being sprayed, start to spray beyond the project, and use brisk sweeping motions to apply a light mist. Wait for the light coat to dry (most paint takes a few minutes with light coats...) and apply the next light coat, and repeat until the paint is the way you want it. Keep in mind that the first coat may start the curing process within an hour, and applying a fresh coat might cause wrinkling. Check the can for info on when coats can be applied. My Krylon says additional coats can be applied within 3 hours and after 7 days. Heavy coats are bad, the paint will drip, the surface will be uneven, etc.
Hope some of this helps.