A beard trim at the shop sets the shape. What you do in the two or three weeks after decides whether it holds.
Leave the cheek line alone
Your barber sets the cheek line to follow the natural growth. If you tidy it at home, take off strays above the line rather than redrawing it. Every time the line moves down, the beard looks thinner.
The neckline is the one that matters
The neckline is what makes a beard look kept or unkempt from three feet away. The usual guide is a curve about two finger widths above the Adam's apple, running toward the back corner of the jaw.
Clean below that line every few days. Do not take it up to the jawbone, that removes the fullness under the chin.
Wash, condition, comb
Beard hair is coarser than the hair on your head and it dries out faster. Wash it a few times a week, condition it, and comb it downward while it is damp so it trains in one direction.
A light oil after a shower keeps the skin underneath from flaking, which is what most beard itch actually is.