First take drain apart in back and bottom, measure new tub and take out tile accordingly slide out old tub and measure tub on wall side from floor under edge and nail 2×4 on studs to support side of tub, it going to be hard to get old tile off without breaking it, I sugest just put design on bottom course. Go to http://www.google.com and ask how to install tub they will have a picture.
Art retired plbg