This is drill #14 in my swim book - Simple Swims: practices for balanced, stable swimming
Stroking and breathing with one arm while the non stroking arm stays at your side. By doing this you can develop your balance, stability, patience, catch, timing etc. It is very challenging but well worth putting in the time to do it. You will likely notice you have a "chocolate and vanilla side"- one side feels more comfy than the other. With this knowledge, continue to practice both sides equally.
Start in the front float with a flutter kick, then bring one arm along side the body. Stroke with the lead arm only, roll/shift your hips as you would when you are swimming, but leave the one arm at the side-but move as "though it is stroking" but it's not. Shift the hips evenly to both sides -not just one side. Breathe only to the side of the stroking arm. Maintain a gentle flutter kick. Cross the length of the pool. Change arms for the next 25.