How to use wind currents for constant elevation in gliding?
You need some inhomogeneity to extract energy. For gliders there are three basic ways to do that:


Thermals: The sun heats up the ground which in turn heats up the air above it. Hotter air is lighter and rises up. It keeps rising as long as the lapse rate of the atmosphere is above the adiabatic rate. This means that three conditions must be met for thermals to occur:
1.1 Sunshine
1.2 Sufficient atmospheric lability
1.3 Ground that easily heats up, such as forest or sand.
Ridge lift: Air streams across a hill and rises up on the windward side. Behind mountain ranges you might also find lee waves, an atmospheric oscillation caused by wind blowing across a mountain.
Dynamic soaring: Here the wind gradient in the atmospheric boundary layer is exploited by flying turns while pulling up when turning into a headwind and diving down when turning into a tailwind. This is currently only used by Albatrosses and has been experimented with by Ingo Renner, but is much too stressful for human pilots if done for extended periods.


Look at the variometer in your glider and try to fly circles when you see an indication of rising air. Tighten the circle when climb rate decreases and flatten it when it increases in order to center the glider in the thermal.