Not sure about when to knot - but you will want a beading needle. The "eyes" of the seed beeds are never the same. You might find a few that will go through a needle - but many will be too small to fit over the needle eye.

If you aren't putting very many beeds on, you could probably find enough that will fit over a fine needle - like a quilting needle.

I looked lost in the sewing dept - I didn't know which needles to use - or what thread . I found out that not all seed beads fit over the needle ! LOL
I think I want to put about 25 beads on the sweatshirt. No big deal but ---

*** I'm determined !

Hi toomuchglass,
Fancy meeting you here! : )
Beading needles are usually found with the jewelry making stuff. They are called beading needles and are often longer and finer than hand needles. You do need to find beads that will fit over the threaded eye so test them.
Are you sewing on beads individually or in groups/small strings? If you sewing on individual beads you can attach them with a running thread. I personally would take a couple of half hitches around the thread under each bead but that's just me. The half hitch won't stop the thread from pulling completely out but it slows it down so you,hopefully, don't lose all the beads.
If you're sewing on the beads on in small strings you can pre-string the beads and then attach the secured string to the garment. Depending on the look you want you can either leave the string loose to dangle or secure it with a few stitches along it's length.
Good luck and let us know ( and see!) how things turn out.
Sylvia

Thanks for the advice .( Hey - are you following me ?? ROFL !! ) It'll be one bead at a time - at least I now know what needle is the best and how to attach it . This is my "Project to do while watching TV " .

****** Please don't let me spill the beads **** gulp ! LOL