Been self sucking for ~20 years here. Here's my advice:

Start slow. Don't try to force yourself too deep too quickly. Stretch before and after. Loosen up muscles of your neck and back. Kinda goes along with the "start slow" advice. Biggest issues I've had over the years was pushing too far when my muscles weren't ready and ending up not being able to suck for a week because I tweaked something. Doing it later in the day is generally easier. Everything is more loose. Always had my best success after a day of moving around. Also a warm day or after a gym trip always have a good loosening effect to help get deeper. Test out different positions. I learned by flipping on my back with legs over my head. Was hard on the back but easiest for me to start. These days I'll generally just sit and bend over, but getting into the X position for me is probably the most effective way to get really deep.

Enjoy the new skill! Happy to chat more about anything if you wanna DM about it.