You insert the gas check facing up in one of the lube grooves and pour as normal. You can see the bullet shape very clearly and with that mold there are several lube grooves from which to choose. Since that mold is for a gas check bullet, the front is "semi-wadcutter". A plain base bullet mold would create a "full-wadcutter".
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"diopter's" idea is just that you have the gas check facing down instead. If the gas check was clean (use a bit of solder flux) I imagne it would stick well enough to remain attached to the bullet after firing. I find that gas checks used as I do remain as I have recovered several from the berm. They cut perfectly round holes in the paper of course.