There's not a whole lot to say. Well, maybe there is, but not from me. ;-)
The above illustration is how I do a square shoulders adjustment on my sewing patterns. I lift the outer edge of the shoulder (usually 3/8" - a little goes a long way here) and also lift the underarm the same amount. This keeps the armhole the same length so that sleeve adjustments are not necessary.
I usually just draw these adjustments on the tissue before I cut out or trace the pattern so I don't have to go back and add tissue for the adjustment later. A straight edge for the shoulder line and a French curve for the armhole are handy.
And that's it.
For sloping shoulders, you would do the opposite. I.e., lower the outer edge of the shoulder and drop the armhole the same amount.