The stopknot that I use is very simply a whipping. There are a few different knots that will
do the job needed,including just 2 or 3 overhand knots if you can't master anything more to start with,but as with the rest
of the knots,this is how I personally tie it.
First make a loop with your stopknot line along your mainline.
Start to make turns along your mainline.
Make about 5 or 6 turns down the loop.
Pass the tag end through the loop
And carefully pull the loop down until it's firm
Then just pull both tag ends tight.
And trim off the tag ends