There are many ways of putting tension into the ridge. Sometimes I use the method demonstrated by Woodlore and illustrated in the Ray Mears book Bushcraft, also as shown in the first part of this Youtube video:
This method is very compact, good if your trees are a little close together relative to your tarp size, but with low stretch line it can be difficult to ensure it is secure. It is of paramount importance that the knot is snugged right up against the tree. Without the tree to close one side of the knot, it will capsize and come un-done.
More often these days I use a modified version of the Trucker's Hitch, originally shown to me on one of Mors Kochanski's courses. It needs more spare distance between trees, but only requires you to wrap around the tree one time, which I find easier and faster.
The main ridgeline is on the left and the working end is held in the right hand. A loop is twisted by the left hand in the ridge. At least three twists, in smoother line, four or more are needed.
Fold the twisted loop back towards the tree around which you just passed the free end of the line and take a "bite" or tight loop of the line through the twisted loop.
Pull this bite as if you were making a chain of loops.
Pass the working end of the line through this new loop.
Pull on the working end to add tension to the line. This arrangement is like pulley, providing mechanical advantage.
To lock off the tension, pinch the bite...
...and slide your finger and thumb to push/pinch the point where the line bends. This is like putting the brakes on, by pushing in the same direction that tension has been applied, tension is maintained while you tie off.
Create a loop, like a figure 4...
...and pass a loop of the working end of the line through.
Pull on the loop, in the direction shown, back towards the rest of the knot, to lock off. Try to pull asymmetrically on the loop so that you tighten the half hitch you just made, without pulling too much of the tail through.
The finished hitch. See how those multiple turns in the original loop wrap around the line, the friction they provide prevents the knot from closing up.
With only one twist at the start you end up with a figure eight loop, which gives slightly less mechanical advantage, and because it lacks the friction to stay open, can tighten up to become much harder to untie when it is time to pack up.
The Trucker's Hitchs is often illustrated like this:
As can be seen, this is basically the same as above, but with a sheepshank type loop locking one end. This additional security does not seem to be needed if you have enough wraps.