easiest approach is to pull the back apart and shave down the styrofoam hump under the vinyl and padding, then for length chop the padding, styro, & plastic pan right after the smc seat base or so and re-staple the vinyl. this will get you what most refer to as a shaved seat.
anymore you will take more effort and skill and most likely have to get re-covered.
best bet would be to find some blaster folks in your area and look over theirs and put on your ski for a test run.