1. Remove the body from the chassis

2. Mask off parts not to be painted. (I'd use Tamiya tape and paints because I've found them easy to use and produce the best result)

3. Spray body with gray primer for coverage. White primer will take several coats to cover black and be thick, obscuring detail.

4. Spray with white primer to allow true color of red paint. Red on top of gray will be dull, off color.

5. Spray with red color.

Good article on paint process