You flex the control surface to full up deflection, you fold a piece of covering (or tape) in half (length wise) and iron it to the bottom of the hinge gap (don't iron tape). Depending on the size or the control surfaces the covering width will vary. On 30% planes I use about 3/8 inch wide strip, on 35% about 1/2 inch wide strips. You may also do about a 6 to 8 inch length then another 6 to 8 inch length, another, etc. until the entire gap is covered. It may be easier for you to work with a shorter strip of covering.

I use clear covering or blenderm tape. I now prefer the tape plus I can get it at any pharmacy.

I'm sure others will have a better description of the process.

Steve