1. Inspect the rim
It needs to be without sharp edges. Straight, without bends.
2. Make sure that rim tape is good and well placed
If rim tape is bad or not placed well, now is the time to fix that. Post on rim tapes: Rim tape types.
Rim tape mustn’t be too wide, to reach walls of the rim, where the outer tyre clinches to the rim. It also mustn’t be too narrow. Or misaligned, to any side. It should cover the inner part of the rim where the tube goes. Just like in the picture above, except that it needs to be without damage, holes or cuts.
3. Place one side of the outer tyre on the rim
The part of the tyre where manufactuer’s logo (and tyre model, if noted) is printed should be aligned with the valve hole on the rim, facing the right hand side of the wheel. That gives an impression of a well done job with attention to detail. It also comes in handy in case of a puncture: once a hole in a tube is located, it is easy to know at which part of the tyre the sharp object had entered (and, perhaps, stayed) – since the position of the tyre on the rim is known.
4. Mounting tube onto the rim, inside the tyre
Lightly inflate the tube, just so it gets less than completely flat. Then put the valve through the valve hole in the rim (secure the valve with a nut, if it has a holding nut – don’t screw it tight, just so it doesn’t fall out).
Now tube needs to be put all the way onto the rim, beneath the tyre. Going round from the valve, towards the opposite end – at both sides of the valve.
5. Mounting the other side of the tyre onto the rim
Paying attention not to pinch the tube in the process, start from the valve and go along both sides. Using thumbs, start mounting the tyre onto the rim.
Critical moment is when the final third of the circle is reached. It becomes harder and harder. When it is no longer possible to mount tyre with hands, hold one side with one hand, so that the tyre doesn’t go off, then with the other hand go all the way around the tyre, pulling it towards the opposite end.
This sounds a lot more complicated than it is. Simply start at one and, then pull the tyre along the rim to the other end. The wire holding the tyre is unloaded and “moved” towards the other side, to put it that way.
If it goes too difficult, make sure the tube is not too inflated, and that it is all seated well – on the other side of the rim. On the other hand, if the tube gets stuck beneath the outer part of the rim and the tyre, then it might need a bit more air.
6. Inflation and checking
Double check all is well seated. Inflate tube slightly, just so that tyre gets round shape. It should still be soft. Move hands around the wheel, pushing tyre inwards, to sit well onto the rim.
Add a bit more pressure. Check again if it is moving off the rim in any place. If it is, deflate a bit, and put it back in place with hands.
Then add a bit more pressure, so that the tyre feels firm. Tyre has an aligning marker near the rim.
If the marker is not aligned all the way round, the tyre is not seated well. If it is deeper in one place, then adding a bit extra pressure to the tyre might make it pop into place.
If this doesn’t help, tyre needs to be deflated a bit and put back into place with hands. Also, if at home, one could use an alcohol based window cleaner spray, to spray a bit between the tyre walls and the rim. It should get back in place on itself when inflated.