A flat turn is basically a turn you can make with losing very little altitude, which is exactly what you want in an emergency situation. They are not hard to do, but absolutely must be practiced before you need them for real.

FLAT BRAKED TURNS

The good news about braked turns is that they are easy to do, and are now covered in the ISP. To do a braked turn, go to about half brakes (or whatever brake setting gives you minimum descent rate, just before a stall.) Then let one toggle up a little bit. This will allow the canopy to turn while descending very slowly. To stop the turn, bring that toggle back down to the same position. Now you are back at minimum descent rate. Try this a few times above 1000 feet to get a feel for how it works. Practice turning exactly 90 and then stopping on heading.
If you ever need to do this in a panic situation, immediately flare to that position. This will slow down the canopy and give you more time to think about it. Then let one toggle up a bit until you have turned just far enough to avoid the hazard, then go back to the flare position. IF, and only if, you have enough altitude, then you can let the toggles back up and flare normally. However, in most cases you are going to have to land with your brakes halfway down (i.e. in an accuracy approach) so prepare to do a very good PLF. You should NOT try to flare any more, since you have already brought the canopy to a near-stall.
BTW the braked turn is also a great way to dodge people shortly after opening, since going to half brakes slows you down so much.

TRUE FLAT TURNS

A true flat turn is a bit different, because it relies on the extra lift your canopy can generate. Your canopy has a limited amount of that extra lift available, and that comes from your available speed. To make the flattest possible turn you have to use that speed to get as much lift as possible – changing your direction requires lift just as keeping you in the air does. To start a flat turn, pull down one toggle then immediately follow through with the other one. If you go to half brakes before you start the turn, you lose some of that speed (and available lift) and thus you can’t make as flat a turn.
To practice flat turns try the following: (all above 1000 feet of course)
1. Toggle turn your canopy, then let go of the toggle. Note the resulting dive and notice how your canopy is now going faster. This is too fast to exit a flat turn.

2. Now flare your canopy and release the toggles suddenly. Note how the canopy sort of “pauses” for a second then dives to recover its lost airspeed. This is too slow to exit a flat turn.

3. Start a toggle turn and immediately follow with a little opposite toggle. Continue with the opposite toggle until you’ve straightened out and the canopy is over your head again. At this point release both toggles. If you came out too fast, try again with more opposite toggle. If you came out too slow, try again with less opposite toggle. Once you get to the point where you exit with normal flying speed you’re in good shape. That means that after you flat turn, if you find yourself at 10 feet, you can immediately flare and get a normal landing. This is the main difference between a braked turn and a flat turn, and can be important if you have a high performance canopy that cannot be safely landed with an accuracy approach (i.e. what happens with a braked turn.)

FLARE TURNS

All the above can help you if the guy cuts you off at 200 feet. But what if he appears heading straight at you after you have started to flare? In that case, you have to flare turn. To do this, flare the same as you always do, but flare slightly more with one hand than the other. This starts a turn. Before your feet touch down you must stop the turn by continuing to flare and by bringing the other hand down farther. The objective is to start the turn with one hand, then keep flaring, and use the other hand to get the canopy back over your head before your feet touch the ground.
This is a useful trick for dodging people who are about to hit you, or whuffos who end up standing in front of you – or even dodging that broken bottle you didn’t see until the last second. Learning flare turns is also an excellent way to improve your landings. Many jumpers “stick out their hand to break their fall” during the flare, which of course turns them hard in that direction. Learning to turn yourself cures you of this, because you learn to automatically compensate for that tendency.
Some tips on doing flare turns:
1. Start the flare going straight, with both hands coming down the same amount.

2. At some point bring one toggle a little lower than the other. Resist the urge to stick that hand out to “protect yourself.”

3. Once you turn even a little, continue the flare by bringing the other toggle lower. This will straighten you out.

4. End with your feet touching down with the toggles both pulled down the same amount.  Don’t give up when your feet touch – keep flying the canopy until you’ve stopped and all your weight is on your feet.
Flare turns can be practiced above 1000 feet, and should be done at that altitude until the jumper is comfortable getting the canopy back over his head. Unfortunately there are no ground references at 1000 feet, and the difference between a good flare turn and a bad one can be two feet. At some point you have to try it during the flare, five feet above the ground. The key to not getting hurt is to start slowly – ten degrees is more than enough turn to start with.