How to wind the string trimmer on my bump-advance string trimmer?

The trimmer string is always jamming.
I wind it tightly, I wind it loosely, I spray silicone on the string but still it jams. Should I change string size? Could the cut-off blade be the problem? I have seen professionals without any cutoff assembly, just the head.
Is there some secret to the universe that only I am missing?

What brand? And I may be able to help more... are you using what the manual and bump head has stamped on it, on the size line?

The make and model:
Homelite UT20022A
25cc String Trimmer
String size 0.80

I followed the directions in the manual, loading 9 feet of string so it tends to be closely wound, not tightly, but still that's a lot to get into the head.
The grass deflector and edger attachment are in place.
I bump and no string advances, I open the spool and the string is caught under several layers, almost as if the deflector/cutter is not cutting the string but grabbing it and pulling on it. I tested the blade and while it couldn't cut paper it still is sort of sharp, enough to sever the line at its high speed, but is it sharp enough? There are two lines coming out of the spool, one line gets worn down and jammed, the other sometimes gets pulled out of the head.

9 feet sounds right.. no the blade that cuts the string on the deflector doesn't have to be sharp...

I'm sure you wrapped it around the correct way (just making sure)
Those pros you see mainly use them without deflectors as to give a bigger cut, and will use heads you just feed what you need in.. it could be time for a new head.

How to wind the string trimmer on my bump-advance string trimmer?

The trimmer string is always jamming.
I wind it tightly, I wind it loosely, I spray silicone on the string but still it jams. Should I change string size? Could the cut-off blade be the problem? I have seen professionals without any cutoff assembly, just the head.
Is there some secret to the universe that only I am missing?
Look on the spool, see if it show's the size string you should be using or look at the owner's manual or go online, if it's electric probably uses .065, if gas powered, probably uses either .080 at least up too much higher like 1.25. If you go online to the manufacturers website, seeing if there are intructions on how to wind the string.

I have the same problem. Following all the instructions, but the nylon 'string' is so stiff it doesn't wind tightly or lay neatly. It springs back and tangles up. When you wind a bobbin on a sewing machine, there is tension on the thread and a guide that makes the thread stack up properly. If you don't use these, you get the same effect -- a loose tangled mess. I considered trying to make sure there is no twist in the line, but that dosen't seem to help. All the instructions out there seem to be written for a 4 year old, and don't go beyond the obvious. But, this is not an uncommon problem!