Secure the etched PCB on a flat surface like a desk using pieces of scrap PCB to border it firmly and tape them down. Then align and tape down the stencil making sure that it is aligned to all the perimeter solder pads around the PCB. Apply solder paste near the solder pad concentrations and use an old credit card or similar flexible plastic to spread & press the paste into the pads' stencil cutouts. A shallow angle (30°) works best with the credit card or you might 'dig' out paste from the pads.

First remove all but one of the securing tape pieces while holding the stencil firmly in place with a finger over a an un-pasted part of the PCB. Very carefully, and in one direction ONLY peel off the stencil.

Now you can proceed to place your parts and hot air solder the PCB!.

For a double sided approach you'll have to completely solder all smd parts on one side, flip the board and reapply another solder stencil/paste etc. because of the thickness of the SMT parts, I advise using additional scrap PCBs, stacked to create a thicker bordering frame to hold the workpiece in place during pasting.