A good pair of glasses will work wonders or even a handled magnifying glass should work to enlarge what you are looking at without blocking the stream of light from the lens. At home I use a Peak 1 grain finder, and it was well worth the investment, it performs fantastically, if you ever come across one at a reasonable price get it!

As for tips, use a bright lens of smallest focal length you can for the format. eg 45mm or 50mm 2.8 lenses for 35mm. Focusing wide opening will give you a bright image to work with. The rest is practice focusing forward and back until the image snaps into focus. Try looking for spots on your image that have text, or detailed edged, which will make it a bit easier to judge how sharp it is.