HOW TO USE A MAGNIFIER
With a large magnifier (e.g. for reading books and maps) hold the magnifier part way between your eye and the object. Now move the magnifier very slowly backwards and forwards until you have the best image, not too small, not too fuzzy. The idea with the largest reading magnifiers is to sit up straight, sit comfortably, you should not have to bend over the magnifier and peer closely (essential for navigating in a car, you need to glance down at the map and glance up at the road signs).
With the most powerful magnifiers (e.g. a jewellers loupe) hold the magnifier as close to your eye as possible (if you wear spectacles you may leave them on or take them off, whichever is the most comfortable). Next, bring the object very close to the magnifier, so close that it's almost touching. Finally, very slowly move the object away until it is in focus.
With 'in between' magnifiers such as magnifiers for the partially-sighted, or folding pocket magnifiers, move the magnifier slowly backwards and forwards until you have the best image, not too small, not too fuzzy (same as above) but move your eyes close-ish to the magnifier. The specialist magnifiers for the partially-sighted by Eschenbach have both measurements printed on them: how far to hold the magnifier from the object and how far to hold your eye from the magnifier. But you don't really need instructions, just experiment, you will find that moving your eye slightly closer or further from the magnifier will make a dramatic difference to what you see.