Recessed can lights have two adjustable mounting brackets that slide out to attach to the ceiling joists. Slide out these brackets and use a hammer to nail the temporary bracket support grips into place. These hold the light in place until you are ready to secure them to the joists. You can fasten these with nails or screws to support the fixture to the joists.
The bottom of the light fixture should be even with the bottom of the ceiling joist with only the circular opening protruding down the thickness of the drywall to be installed, so that a flush finish is achieved. To check this, use a scrap piece of drywall and hold it up along the side of the circular part of the can light. be sure that the light doesn't hang lower than the bottom of the drywall.
The next step will be to wire the recessed can and install the drywall. Once the drywall is installed, the fixture will need the trim ring that houses the bulb and this will attach with springs. It will secure firmly and flush when installed correctly. The can light trim should cover the hole cut in the drywall and not have a gap showing when installed.