Good for you! Thread the flywheel nut down until the top of the nut is flush with the top of the threads. You have the puller correctly shown. Back off on the nuts above body (thread toward heads of bolts) and the nuts you have against the flywheel in the picture back upward to just below the body of the puller. Begin to tighten the self-threading bolts (has been a 3/8" socket in the past.) They must cut cut deep enough to grip reliably (5 -6 turns) without the nut touching. Once the bolts are fastened in securely, run the nuts down to the puller body with your fingers until snug and puller is flat on the nut. Now tighten one nut 1/4 turn, then the other 7/16" wrench.) Do this until flywheel pops and can be lifted; then the bolts can be taken out, puller lifted off, flywheel nut taken off and flywheel removed.
Note: the flywheel brake (safety bail) needs to be tied off for this. And note that the two nuts on the side of the puller nearest the thread-cutting ends are only there to keep the bolts from falling out of the unused puller and should never be tightened against anything.
Keyways can be cleaned and flywheel put back on crank. Line up the slots in flywheel and crank and insert key. Tighten flywheel nut to 55 ft.-lb. (safety bail no longer needs to be tied.) Reassemble and mow.