what plane? what gear?
if there air gear, you can pump them up to extend, and jamb the actuator rod into position either with a screw or with epoxy, if its exposed.
if its the enclosed type then open the cases and remove the air piston and just pour epoxy into the case permanently extended gear, but useless for retracts again.

for mechanicals, there is a rod that exits the gear that the pushrod connects to,unscrew the plastic connector on this rod, pull this rod so the gear is fully locked in position, slide the coupler over the rod and tighten.

when asking a question like this it always helps to provide a photo for clarity,BTW I like retracts they streamline a plane if its meant to have them, less drag.