Seek professional help. This is a guide to get your mental gears turning. There is no substitute for actual instruction.
A bit of theory first.
We do not have flat wings. The angle of attack is greater in the center of the wing than at the tips. This concept is paramount since flaring a glider is stopping it by stalling it.
Stalling the root (center region) of the glider without stalling the tips does not produce good results. In higher winds, this ends with the pilot trying to catch up to (and slow) the glider as the pilot touches down. In light or no wind, the glider noses over and smashes in on it's nose. This is often referred to as a "whack" due to the noise it makes.
The reason for this is hang glider design. That twisted and swept wing is built to nose over when you stall it. This is a wonderful characteristic when flying, but makes things difficult when landing. The center (root) of the glider reaches a stalled angle of attack before the tips. If pushed to a stall slowly, only the root will stall. Since the wings are swept, the tips (which are still producing lift) lift the back of the glider, nosing it over.
To avoid this, you must stall the tips. To stall the tips, you need to push the glider to stall quickly enough to push through the root's stall with enough momentum to get the tips to stall.
When to flare
"Flare timing" is critical and it very dependent on how aggressive the flare is. Remember, you need to push the glider through the root's stalled AOA in order to stall the tips. As you approach the stalled AOA, the root produces more and more lift. The slower the flare, the more time you allow the wing to produce lift. This can, and often does, result in pilots climbing when they intended to flare. As they climb, the glider looses speed and the root reaches stalled AOA without the tips reaching it. The results hurt.
So, you want to know for certain that you can flare the glider fast enough to stall the tips without allowing the glider to climb first. There are a few techniques for this.
This is the most commonly taught technique and perhaps the easiest one to master. The idea is to get the glider into ground effect at a higher speed than trim. As you skim the earth, you will naturally slow the glider trading speed for lift. When the glider hits trim speed, you flare.
This works well for properly trimmed gliders and "normal" flares. With higher performance gliders (Intermediates and Toplesses), it can be useful to count "One" when you hit trim before flaring.
The Two Step Flare
If you're flying outside of a gliders weight range or have it trimmed unusually, the "trim flare" may not work for you. The answer is the Two Step.
Again, you get the glider into ground effect with good flying speed. As the glider slows you determine when you're approaching a flareable speed. You test your guess by pushing out an inch.
One of two things will happen at this point. The glider will climb. Or it will not.
If the glider does not climb, flare. If the glider does climb, hold your test push and wait for it to stop climbing. When it stops climbing, flare.
As you can see, there is a bit of "guesswork" involved, but when used properly this technique works with any glider.
I'll let someone else fill in the blanks like form and technique. (maybe I'll get to it if I have time) But the best form or technique will not help you if you don't know when to use them ;)