My first rule is, if you are having to completely rethread it, is never rethread it at night! I do so much better during the daylight hours. Easier to see the dots.
Mine is color coded and that is a big help.
I try to get by with cutting the old color and tying on the new color. I cut one color at a time. Be sure to double knot it and then pull it tight. The knot can't be very big because you want it to sew through the needle.
Then I start sewing on a scrap. When I start seeing all the new colors on my scrap, I know that it is now rethreaded. Hope this makes sense.
If all else fails, I read the manual. Oh, when rethreading from scratch, loosen the tensions to zero, then when you are done, turn it all the way to nine and then back to your original number. I make a note of where each tension should be set. I wrote it in my manual.