Loosen the locking screws on the focusing scale on the front element. You don't have to unscrew them completely, because they are tiny and easily lost. Then put the camera on a tripod, open the shutter on T and with the aperture fully open. Then put a small groundglass (a flat piece of transparent plastic, like plexi glass or cut out of a cd cover, and as a matte surface you can just use scotch matte tape). Then visually focus on something at infinity, buildings or lamp posts far away, by turning the front element. When you have found the infinity focus you can make a scratch on the side of the front and second element lens mounts, so you know where it is in case you disturb it later. Then take the camera off the tripod and put the focusing ring back on in the now correct place and tighten the lock screws.