I got three, they all seem to work on me, but sometimes I prefer one over the other for no clear reason.

1. Counting my breath duration. Breath in at normal speed, count how long it is, then breath out slower than that by two or three counts.

2. Force my thoughts to become disorganised. I do something like free association between concepts and pictures of the inner eye. Common starting point for me is a free flight over a hilly landscape, then random things, woods, trees, rocks, water whatever, I don't try to control anything about the theme. If I start thinking coherently or about something concrete from my life, I just start again, with another nature scene.

3. Imagine a calm scene. The suggested starting point I was told was floating on an air matteress in an alpine lake (helps that we know those around here, but I'm sure non-alpine lakes work too) and imagine the things you can see uphill as you drift around your axis.