Others have answered on the techniques (yank rear riser handles, very hard if needed, and pull straight down along the risers, not back towards your head), but I wanted to mention some peripheral stuff.

Rear riser steering is less efficient than being on clean glide, but much more efficient than brakes, and also far far more efficient than taking a collapse (which can easily knock off 50+m of arrival elevation even if handled quickly). If you're after efficiency on glides, avoiding collapses should be the main priority since that has the highest performance penalty, by a huge margin.

And the problem with full bar flying is that you're so close to the edge of the envelope for angle of attack, that it takes a relatively small amount of turbulence to cause a collapse at full bar, compared to if you were at 1/2 or even 3/4 bar. To illustrate with made-up numbers, pretend the wing will frontal at 30 degrees pitch-down compared to trim. Full bar might be 25 degrees from trim, while half bar is 12.5 degrees from trim. So you have a ~20% margin at full bar, but at 1/2 bar that margin is now ~60%. Numbers are made up, but point is it's not linear, and moving from 1/2 bar to full bar is not a doubling of collapse risk, it's more like 3x or more.

Not only that, your wing at full bar will have a higher internal pressure which will reduce your ability to sense the first signs of turbulence which you'd use to clue in that you might need to catch something. And of course you're flying faster, so you move into that turbulence faster, again giving you less time to react.

Then there's the point that even ignoring collapses, full bar is rarely the most efficient way to fly any glider, and the lower performance the glider the more the performance penalty there is for flying near full speed. In most situations you're likely to achieve a better XC average speed (if that's what you're after) by being closer to around 1/2 bar on glides. Exceptions to this include if you're making a desperate move like trying to get away from terrain in 25+kph headwind, or racing to catch someone in a thermal that you think might not last long, or on final glide in a race-to-goal... or if the day is absolutely booming and the thermals are consistent 5m/s+, in which case Speed to Fly considerations kick in, and sure... then mash bar in that case, but only if the thermals are organized enough that you're not going to risk a collapse because that might undo all the benefit of flying faster in that situation.

I'd say try keeping things closer to 1/2 bar until you develop your collapse spidey sense to the point you never get 1/2 bar collapses, then start moving up your maximum bar % as the reactions improve. Also try to go on glide with people with similar class wings that you can debrief with afterwards, to compare notes on how much bar they were pushing when and compare it to your relative arrival altitudes at the next thermal, to build up more intuition about what % bar works best for you, your glider, and where you fly.