I don't have a letterboxd account, which I'll address later.

From my perspective the task is to find critics whose taste aligns with yours so they are a good guide for films you'll like and films you won't. (I know that seems obvious, but some approach criticism from a different perspective, hence my qualification.) Then, still, you should read a couple random critics to a) be exposed to ways of looking at things you might not, b) find another critic you like.

The reason I feel similar taste is much more important than someone who is a great intellect or a great academic critic is the latter is more likely to concentrate on some little thing that interests them, but is irrelevant to you. After all, even they are merely describing their taste. Yes, there are a few things in film that are objective, such as the quality of the shots, a great set-piece, good script, overall quality and so on, but at the end of the day, it's about taste.

For example, two critics were arguing over whether kids should see The Return of the Jedi - I don't remember who they were. The one was saying 'of course,' but the other said that Jedi was such drivel, they should take the kid (like about 8 years old) to see Tender Mercies instead. I've seen Tender Mercies. It's a merely OK movie that only adults would even understand. Thus the critic who recommended that exposed himself as an out of touch loon, to be avoided. (He was a very revered critic in NY.)

I think the sole value in critics is to talk you into seeing a good movie you might not have, and it's never to prevent you from seeing something. To put it another way, only pay attention to positive reviews on films you're on the fence about. Disregard bad reviews out of hand - because of the taste thing.

It's probably easier to find critics you align with these days due to Rotten Tomatoes. Find three of your favorite movies and then find critics who agreed with you. Then see if you agree with what they thought was crap. If you align, then keep an eye on them.

My two favorites are Peter Travers and Stephanie Zacharek. Ebert was my fave when he was alive, along with Siskel.

I think it's also important to know genres and subjects you're just not going to like. This is not to avoid them entirely, but to not waste your time on the mundane ones. For example, I avoid scary movies, disease movies, and spinach movies (with a blatant moral or sjw slant). Gangster movies and war movies have to be a cut above for me to truly like them, as the topics themselves are abhorrent to me. (Note this is just my taste, and yours may be completely different, and that's OK.)

The reason I don't have a Letterboxd account is that, for a period of time in my life, I saw every movie that was available to see. That period was about 3 decades, though closer to 4. I worked in movie theaters from the age of 14, had a minor in film at college, and then worked in video stores as well. There was a point around the year 2000 where I'd literally seen all movies available in America. Since then, I've gotten a lot pickier about what I'll spend my time on because I realized the above and no longer felt is necessary to see all movies (even ones I knew I wouldn't like) to be versed in film and to make sure I saw all the good ones. I was that guy who'd seen every movie nominated for an Oscar every year. I no longer am, and I find I enjoy movies even more. (Thus a Letterboxd account would serve no purpose for me; it'd just be homework.)

Hope this helps.

tl;dr: find critics on Rotten Tomatoes you agree with on both the good and the bad, so things don't get ugly.