There is a flattened section of the spool directly under the leading edge of the clamping Spring. The end of the film should slide under this. Just push the first few mm in, move your index finger and thumb back a little, and push in a few more mm. Shuffle and repeat until the leader is as far in as the clamp will allow, usually a couple of cm. Make sure the edge of the film is flush against the end flange of the spool. If there is a gap, gently push the film sideways to fill it.
I put the film in my left hand and and spool in right. I am right handed, and this works better for me. Remember, emulsion side faces you (you are lens side).
All this assumes the standard Leica spool.
Good luck!
Cheers,
J 🙂