Decide your budget and non-negotiables, set the auto trader filters accordingly, and see what comes up as a starting point.
I changed my car earlier in the year, my criteria other than budget were: boot capacity (at least as big as I had, preferably a bit bigger), not above a certain length/width (park in lots of small car parks for work), not electric, decent headroom in the back (teenagers), max age, max mileage.
Once I had a list of a few options, the Carwow YouTube channel has some really good videos looking at different cars, doing comparisons between three that are similar, finding the best cars for certain purposes.
I also found a dealership that had a couple of the car models I was considering in stock so I could see them in person (and was very open that I was not buying that day, literally wanted to look at them, and would have walked away at any hint of pressure).
Gradually the list was narrowed down as I looked at them in more detail/spoke to people - eg one model felt really basic and cheap inside, one model I discovered certain engines had a record of cam belt issues, another car a few friends had had and all reported it was really rocky to drive/steer.
Took a couple of weeks but I found one that ticked all my boxes, and I actually love, for all that it was the ‘sensible’ option that ticked the boxes. (It’s a Seat Arona - not too big externally, but excellent internal space and a decent boot, Seat reliability is good at a decent price point).
Good luck!