you just let them sit and dry out.
about the inside 3/4" is the porous soft part, it's similar to bone marrow.
to stabilize antlers you give them at least a 90 day drying period, everything dries and shrinks and tightens up.
to make a knife or something else with a antler handle you need to boil it to get that inner part soft and somewhat flexible again, then you insert the metal while it's hot and soft, then you just let everything cool in place.
if you use a flat longer tang it will be in there about as permanent as using a 2 part epoxy or JB weld.
we make all kinds of stuff out of antlers.
knife handles, candle holders, lid lifters for Dutch ovens, utensil handles, marshmallow forks, bolo ties, coat racks, hangers for my garden hoses, buttons, I hang stuff in the Garage on them, use them for decorations around the garage windows, all kinds of stuff.
I even made the bottom half of a fishing pole handle from a long straight deer antler for a kid I was mentoring some time back.
never made pistol grips from them though.