How to apply a custom plate:

1. Choose a template you like from the "CUSTOM PLATE TEMPLATES"

2. Open template in any image editing software (Photoshop, Paint.net etc)

3. Write whatever you want (I.E FC1988) 

4. Save template wherever.

5. Open up TXD for your car of choice (i.e Adder) with TXD Workshop

6. Find license plate texture

7. Select "replace"

8. Find custom plate

9. Save

10. Go and load the car you wanted the plate for in GTA SA, and be happy. 