There's a point in every changing room where you shove your foot into a Chelsea boot, feel that elastic side panel give, and know within about two seconds whether it's going to work. If you're tugging at the back loop for a solid ten seconds and it still won't settle over your ankle, that's not a stretching in situation, that's the wrong size.
Get the fit right by paying attention to the width across the ball of your foot rather than the length, because Chelsea boots are cut close and a lot of the comfort comes down to whether that elastic gusset actually has room to flex. Too narrow and you'll feel it pressing within an hour, especially walking anywhere with a bit of pace. Too loose and your heel lifts with every step, which no amount of thick socks fixes. The ankle opening matters too. If you've got a bit more calf, check the boot doesn't dig in a straight line across the front of your shin when you sit down, because that's the bit that'll rub on a long day. Go for a proper leather or suede over anything with a stiff synthetic upper, since leather softens and moves with your foot after a few wears, while the cheaper stuff tends to hold its shape and bruise your ankle bone instead of yielding to it.
If you own none, start with a plain black leather pair with a low, flat heel and a rounded toe. It's the version that goes under jeans, under a skirt, under tailored trousers, without ever looking like it's trying too hard. Before you buy, check the sole isn't a thin, hard plastic that'll slide on wet pavement, press your thumb into the elastic panel to feel whether it's got real spring or whether it's gone stiff from sitting on a shelf, and run a finger round the inside seam at the heel, because that's where a badly finished boot will rub blisters into you by lunchtime. Have a proper look at the stitching round the sole too, loose threads there are a sign the rest won't last.
Once you've got a pair that fits properly, they sit happily under straight leg jeans, with a midi skirt and thick tights in the colder months, or with a plain shirt and trousers for anything that needs to look a bit more put together without trying. They're the boot I reach for when I can't decide what else to wear, which says more than any amount of praise could.
Stuff a cedar shoe tree or even rolled up paper inside them when they're not on your feet, it keeps the shape honest and stops the leather creasing in the wrong places.