One important point to understand is that the speed of a glider through the air - its airspeed - and the speed of a glider over the ground below - its groundspeed - are usually different. If there is a steady wind, the glider is carried along with all the surrounding air and it effectively feels no wind at all, only the slipstream of its motion through the block of air. Only if the wind is gusty or rises or drops sharply does the glider 'feel' the difference and there any noticeable effect on the glider's airspeed. It is a common misconception that a glider needs wind to fly like a kite. But a glider is simply an unpowered aircraft and flies happily with or without wind. It simply loses height unless the air within which it is flying is rising faster.
So the speed of a glider over the ground is the combined effect of the glider's airspeed and the windspeed at that height. If the glider is flying at 50 knots directly into a 20 knot headwind, it has an airspeed of 50 knots but a ground speed of only 30 knots. If the wind slowly dies away the glider simply compensates by sinking a little faster to maintain the same airspeed and you don't feel the difference unless you are flying very low. Flying in the opposite direction, the airspeed remains 50 knots but the groundspeed is 70 knots. The biggest consequence of this is navigation - wind drift can carry the glider quickly away from the airfield if you do not keep an eye out for it. It can also cause some disorientation when flying low when you turn from a headwind into a tailwind - there is a rapid rise in groundspeed even though the airspeed remains roughly the same. This can cause the unwary pilot to raise the nose in a reflexive attempt to reduce the speed: which can make the glider fly dangerously slow, risking a stall and spin accident.
The airspeed of a glider is normally controlled by the aircraft's attitude (angle of pitch). You pull the stick back to raise the nose in order to slow down, or you push the stick forwards to lower the nose in order to speed up. To select a particular speed you push or pull the stick and hold it at the position you expect for the required attitude, and wait for the airspeed indicator to catch up a few seconds later. So you adjust the pitch until the correct speed is obtained and then hold that pitch.
To maintain the chosen pitch & airspeed the use of the trimmer is important. The trimmer (or trim control) adjusts small tabs on the elevators in the horizontal stabilisers of the tailplane. The trimmer is used to avoid the pilot needing to maintain pressure on the stick to maintain the chosen attitude. Whenever the required attitude is achieved, the trim should be adjusted to reduce the stick pressure to zero. Then if you take your hand briefly off the stick, or accidentally change the stick pressure, it settles back again so there is no permanent change of attitude hence speed. On the Puchacz the trimmer control is a slider on the left wall of the cockpit.
When I first tried trimming, it was to set and maintain the normal cruising speed. At the cruising attitude the glider should maintain the optimal speed to maximise the glide ratio - about 45 knots in the Puchacz. We were flying faster than this, so I gently eased the stick back to raise the attitude of the glider until the 'correct' amount of ground was visible above the nose, below the horizon. Each person sees a different horizon position depending upon their height and seating position. You soon get used to the view for you personally. As I held the stick back slightly the airspeed slid slowly back to 45 knots, then a little slower, so I moved the stick a little forwards and held it again and the airspeed slowly rose and steadied at 45 knots. Holding the stick still I gently slid the trimmer knob backwards until I felt it try to move the stick, then I moved the trimmer knob a tiny bit forwards again - and that was it set. With only my fingers resting on the stick we flew on steadily at 45 knots.
Every so often depending upon the flying conditions you have to make small adjustments with the stick and follow up with the trimmer to remove any pressure you need to maintain on the stick. Ideally whenever you have to maintain pressure on the stick, e.g. when circling steadily in a thermal, you should set the trimmer to remove the stick forces and make the aircraft easier to fly.
I then practiced changing the speed and setting the trim to maintain it. I accelerated to 55 knots and re-trimmed, then 65 knots and re-trimmed, and finally back to 45 knots and re-trimmed again. It was clear that you should avoid 'chasing' the airspeed indicator while adjusting the attitude - there is a definite lag from the time when the new flying attitude is set and the glider either accelerates or decelerates to the new speed accordingly. You first set the attitude with reference to the horizon, and hold it and wait, all the while glancing at the airspeed indicator. You shouldn't stare first at the airspeed indicator and try to fly it instead of the glider.
A critical time to get the trimmer right is after entering the circuit when the correct circuit speed should be selected. On a calm day the approach speed is 55 knots for the Puchacz. (On a windy day it could be anything up to 70 knots). It is good practice to speed up a little when you are below 1000 feet: for the Puchacz with a textbook stall speed of 37 knots, and a normal cruising speed of 45 knots, you don't have too much spare speed (i.e. safety margin) in the event of encountering turbulence or a strong wind gradient. So below 1000 feet, down to circuit height, maintaining an airspeed of 50 knots is recommended.
Another influence on airspeed is the airbrakes, used to reduce penetration or steepen the gliding angle. When the airbrakes are extended, there is increased drag and the glider slows a little. So as you extend the airbrakes, you normally need to lower the nose slightly to maintain airspeed. Likewise, when you retract the airbrakes you normally need to raise the nose slightly to maintain airspeed. Another effect of the airbrakes is to increase the stall speed, which you need to be aware of if flying slowly. Airbrakes are usually very important during the approach and landing.