nikon d3100 dslr...focus at monitor

[fallia]

i want to look though the monitor at the back of the camera, zoom in on the view and focus....there is a manual setting for focus but i'm not sure how to work that....i'm going to be using a tripod soon to slow the process down and i want to make sure i'm getting the focus correctly weather its in the center or off center (area of interest i want in crisp focus)