The hem is the thing that catches people out. Wide leg trousers drag along the floor if they are not cut for your height and your shoes, and a dragging hem means a frayed hem within a month, soaked through on a wet pavement, trodden on the back by your own heel going up stairs. I ruined a pale grey pair in about three weeks of daily wear before I worked out that the length only works with the exact shoe you tried them on in. Despite that, they are worth the faff. Nothing else makes jeans and a plain top look like you've made an effort without actually trying.
If you own none, start with a straight wide leg in a fabric with some body to it, a proper cotton or a ponte, not anything floaty. Floaty wide leg is gorgeous but it moves when you walk, catches on bag straps, and shows every crease the second you sit down for an hour. A sturdier fabric holds its shape from the waist down and looks tidy even by the end of a long day, which matters more than the swishy version ever will.
They sit nicely across most of the year, which is more than I can say for a lot of trousers. A heavier wool blend wide leg carries a coat and boots through fairly cold months without looking bulky, because the leg is already doing the visual work. Come warmer weather, a linen or cotton version in a lighter colour does the same job for an evening out, or for a working day that starts at a desk and ends somewhere you'd rather not be in jeans.
Fit is where most people go wrong, and it's almost never about the leg, it's the waist and the rise. Wide leg trousers are cut generously below, so brands often compensate with a higher rise and a slightly looser waistband, which on me gapes at the back the second I sit down. Try sitting in them in the shop, not just standing at the mirror. Check the crotch doesn't sit low enough to bunch when you walk, and check the waistband doesn't need a belt the day you buy it, because it will need one eventually regardless.
The leg width itself should skim rather than cling at the widest point of your thigh, with enough room that the fabric falls straight rather than tenting out to the sides. Too much width there and the whole trouser reads as fancy dress rather than tailoring.
Can I sit down in these, walk up a flight of stairs in these, and still see my own shoes at the end of the day?