You can tell you have reached the end of the film by the frame counter and the film advance lever. Either the film advance lever will not turn at all or it will not turn all the way. First press in the rewind button. You may immediately let of of it.

The turn the rewind crank in the direct of the arrow until the frame counter, which counts backwards as you rewind reaches "S"

Then open the back cover, remove the film cartridge, place it back in its canister and have it developed as soon as possible