It's easy.
1. Determine how much mounting depth is available between the door panel and the speaker mount (usually the inner door skin, but this can vary from vehicle to vehicle).
2. Determine how much mounting depth is available between the speaker mount and anything underneath (towards the outer door skin) that a speaker might interfere with, like a window actuator.
3. One you figure those out (especially #1), you can then determine how thick the MDF baffle/adapter can be.
4. Take the MDF, place your stock speaker on it face down (inverted with the magnet facing up), take a pencil and trace around the speaker and mark the mounting holes.
5. Remove the stock speaker from the MDF and place the new 6.5 speaker face down inside the tracing. Break out the trusty pencil again and trace around it and mark the mounting holes.
6. You may have to modify (widen) the 5x7 tracing on the long oval portions to allow more MDF material if the 6.5 comes too close to the 5x7.
7. Use a jigsaw to cut off the excess MDF inside the 6.5 tracing then around the 5x7 tracing.
8. Use an appropriate sized drill bit to drill out the mount holes for the 6.5 and 5x7.
9. Mount the 6.5 to the baffle, then mount the baffle to the speaker mount.
10. Voila! Done.