Step by step instructions for astro-type photos:
1. Choose a time when the Moon is highest (nearest zenith). This is to get the least atmospheric muck in the way. Makes a huge difference with focal lengths > 1000mm.
2. Use a loooong lens plus as heavy a tripod as you can lay your hands on. To fill the frame on full-format 35mm with the Moon disc requires 2000mm focal length, 1250mm for APS, etc.
3. If you are focusing manually, then you have to make several attempts. Focus, grab a few frames, refocus, grab another bunch, etc. Invariably one will be sharper than the rest. If you are using a Canon lens with a focal length > 100mm you'll probably be best off using autofocus with the cenral focus spot only. I found this to work very reliably, even with stacked tele-extenders when Canon says it should not.
4. Exposure. Most tripods are too flimsy for looong focal lengths. Use high ISO and high shutter speed. Your lens needs to be closed down only to get to its optimum aperture since depth of field is irrelevant. After you have success at high ISO you can venture into lowering ISO to a level that your tripod can handle. Mirror pre-release is of course assumed. If shooting digital you can easily figure out exposure from the histogram. If shooting film use the Sunny-16 rule as a start and bracket in half stops to longer exposures. Some mirror lenses/scopes lose a lot of light, requiring opening up a stop or even more. If the Moon is at 50% phase (Half Moon) double the exposure for Sunny-16, Quarter Phase requires quadruple Sunny-16, etc. With bracketing for exposure and "bracketing" for focus do not expect more than one decent shot from a 35mm roll. Here is an autofocused bunch of crops using a Canon 600mm f4 lens wide open with extenders (yes, extenders do work very well, despite the nasty rumours you may have heard):
The crop at 1680mm is at 1:1 (100%), the one at 600mm is at 280%, etc.