This is from the Service Manual for CRF 230 F (03-05) but probably is true for other years:
REMOVAL
Remove the nuts, collars and bolts (from under the rear fender)
Slide the seat backward and remove it.
INSTALLATION
Install the seat by aligning the groove and prongs with the boss and hooks. (there are 3 such points)
Install the bolts, collars and nuts, then tighten the nuts securely.