Half-halting or checking back is simple; I do it a lot if my horse gets too fast. ;)
You simply take the reins (usually in both hands) & slowly pull back, then release. Pull back, release, until she slows her trot a bit. Don't use too much force pulling back remember; you just want her to slow down her pace. But just enough to have a bit of contact on the mouth. :)
Hope I explained it well!