How should I trim my 35mm film so it will always load?

The traditional view is that LTM cameras require a much longer and thinner film leader to ensure proper film loading. Thus John Black notes:

You really don't need a template (Leica ABLON) to trim leaders for screw mount cameras. I just use a sharp pocket knife [ or scalpel or scissors ] and cut the existing leader in half (down the middle) for at least 4 inches and then ramp the cut up to the top edge. Be sure NOT to cut through a perforation but between them. The shape of the leader does not have to be precise. I have an ABLON but have never used it because the [ cut-by-hand ] method works well every time. [ … ] I trim all rolls of film for the day or trip before I go out to shoot (saves time).

However, others remark that you don't need to bother with a long take-up tongue at all, just wrap one turn of film around the take-up spool and then drop it into the camera. Quite a few users report this technique has worked well for decades.

Another clever idea is to use a thin piece of plastic or cardboard to protect the back of the shutter curtain during loading. Usually with LTMs you have to open the shutter at "T" and keep it open so you can watch and if necessary reach in throught the front of the camera during loading. By using (say) a phone card you can skip this and just load film (almost) normally.

How to Load LTMs

See the following web page by Christoph Ozdoba for photos and examples: