Hi hi
Yeah... Ok... So...
Hook your sling onto something. I use a door handle.
So let's say you're braiding a 7 strand braid. Your instinct might be to pull all 7 strands hard and hold them tight all the time during braiding. You should try to ignore that instinct.
You have 4 strands on one side and 3 on the other, and you're about to pass the outermost strand over to the other side. Before passing the strand over, grab it and pull hard on that individual strand against the door handle (or what ever you have hooked your sling on). It is VERY important NOT to hold any tension on the other 6 strands. The tightness comes from the contrast between that one strand you're pulling and the rest of the braid you keep loose. When you do that you will see the rest of the braid "snake" or "twist" or "zigzag" left and right around that one strand you're pulling for like 1 cm up the braid. It will look chaotic, but that's exactly what you want.
If you do that, what happens is that you're braiding the sling at the point you're passing the strands over, but it is tightening about 1 cm further up the braid. You will see the strands wrench against one another so tight the thread will lose color and go paler.
I strongly recommend using a good pair of work gloves, the kind where the fingers and palms are dipped in rubber. Braiding with gloves like that is going to feel terrible, but you will be able to pull MUCH harder, and you will protect your hands. I guarantee that you will not be able to braid the entire sling sufficiently tight without seriously hurting your unprotected hands.
I also suggest taking a few offcuts of thread and testing the method out. I still have a half finished sling in my cabinet with half of the retention cord floppy. It took me half the retention cord to get the technique right. By then I had to start over.
Hope that helps.