There is a YouTube video that shows a couple of ways...she demonstrates a 'starting' method which may help you at about the 2:50 (time) mark...but watch the whole video:
When I first started shaving/trimming their ears, I held them under my left arm to secure/steady them, and used my left hand to hold the ear stationary...as demonstrated by this video, and, yes they wiggled some and I had to re-establish my grip. I, too, used an Oster Finisher Trimmer...which I love for this purpose, and they were 'uncomfortable' with what I was doing to them at first, but now they mostly take the process in stride...much as does the Parti shown in the first part of the video...which is how I trim their ears now.
Good luck.