The key to good trim is perfect measurements, right? Well, partly. No matter how often you measure a distance and transfer your measurements to stock, you have a margin for error.
So wherever possible, hold the stock in place, and mark it more accurately that way.
You can actually case a door or window without using a tape measure at all. Start by marking the reveal (the edge of the jamb that you’ll see when the casing’s installed) with a pencil. Since the measurement is consistent (maybe a quarter inch), a good way to do it is to set your combination square so only a quarter inch of the extends beyond the right angle. Place your pencil on the end of the and slide the and pencil along the jamb.
Next, place the left-side casing against the jamb and mark your cut at the reveal corner. Miter this piece. Move on to the top piece. Miter one end, hold it in place against the installed leg, and mark the cut at the next reveal corner. Then hold up the right-side casing to the installed top piece and mark it. Cut your miter.
You could also do both side pieces first. Then miter the top piece at one end. Hold it upside down over the two side pieces, and mark your cut on the top of it.
