Two minutes with a tape
You need two numbers and a soft tape. Getting them slightly wrong is the most common reason a chart sends someone to the wrong size.
A soft tailor's tape. If you do not own one, a piece of string and a ruler works: wrap the string, mark where it meets, lay it flat and measure the marked length. A metal builder's tape does not bend around a body and will read short.
Measure over underwear, or over whatever thin layer you would wear under the piece. Measuring over jeans adds an inch and a half and quietly moves you up a size.
The narrowest part of your torso, usually about an inch above the navel. Not where your trousers sit, which is lower on most people and a different number. Stand normally, breathe out, keep the tape level all the way round. Do not suck in; you will have to wear the thing while breathing.
The fullest part of your seat, feet together, tape parallel to the floor. Look in a mirror for this one if you can. The tape rides up at the back more often than not, and a tilted tape reads small by an inch or more.
Snug enough that the tape stays put when you let go of the end, loose enough that you can slip a finger under it. Two people measuring the same waist can land an inch apart purely from how hard they pull, which is worth remembering when you are one inch from a size boundary.
Check what the waistband is. A drawstring or a full elastic band means the smaller size will still close, and you can pull the excess in. A fixed band with a zip means the smaller size is a gamble. When in doubt on a loose cut, the smaller one usually looks better, because these patterns already have room built in.
Some of our tables carry a bust column and most trouser tables carry inseam or total length. Those two are garment measurements: they describe the piece, not you. Measure a pair of trousers you already like, lay them flat, and compare the inseam against the table if length matters to you.
Once you have the two figures, the fit finder will hide everything that is not charted for them. It reads 51 tables at once, which is the part that is tedious by hand.
