It's simply a question of having good hydraulics.

While spinning is fine for normal riding, at some point you have to be able to produce torque and that comes down to powerful quads, which you'll only develop by climbing in big gears. I'm not arguing for riding using big gears and strength vs. developing speed for normal riding. That's been settled for most people long ago, speed is faster and easier on the body.

But you want to have the option of using big strength when and where you need it, especially if you ride SS or relatively narrow gearing.

So my advice is simple, go out and find the hills, find the steepest ones you can climb at all, and the longest ones you can get to the top of, and build them into a normal routine. When they get easier -- and they will -- look for steeper and longer.

There are also some specific techniques that will help overcome steep hills.

One is to slalom up, which lengthens the climb, thereby making it shallower.

Another is to push back on the saddle, which is like raising it slightly, and to use your ankles to increase the length of the power arc. Since you only produce effective power for 2 arcs per rotation, lengthening the arc means that less force is needed at the peak to produce the same amount of power output.

Lastly, drop your ankles at the top of the stroke, then recover them using your calves as you pass the horizontal. This opens the leg somewhat at the top of the stroke improving the leverage through the knees. It also angles your foot to better power through a longer arc, pushing forward and down to start, and hback and down to finish.

Note, that these are different pedaling techniques that will not be practical for steady riding, but can be another tool in your kit, to be pulled out when needed.

But in the final analysis, single speed or multi geared, climbing is about power and torque, and you only get these by climbing hills that seem too tough to climb.