I have two ways, depending on the context.

My preferred method is to not introduce my vision impairment. It's obvious I have vision impairment, if they want to know about it, they should ask. Of course most able bodied people are very nervous about making enquiries into a person's disability. This leads to Fun. Feel free to go ahead and ask them for any assistance you might need and so forth too. Don't explain why unless they ask, it'll spoil the Fun otherwise. (I suspect most on here would disapprove of this approach. I'm happy to elaborate more extensively why I'm fond of it though if you'd like to know more).

The other way is to state very promptly, "my eyes are shot so...". The "so..." part is where you follow up with something like "so when I ask for some information from [somewhere], you need to read it to me", or "so I might find it difficult to do [thing you've assembled to do] in these conditions". The most recent example I have is session zero for a new D&D group where I set out that when I ask for some information on say a spell etc, I need them to read it from the Player's Handbook for me (I'm not incapable of doing it myself, it's just harder and more time consuming, so better they do it).

Actually it's not bad to say it.

I work in store and one day a girl came buying some stuff,she was wearing glasses and made her way through the store with so much confidence that I didnt notice that she has problems with sight. So as I was showing her stuff she got annoyed and said something in line of why am I showing her this stuff since she cannot fully see it.

I honestly didnt notice that she has problems with her vision. If she had mentioned it,it would be much easier for both of us. She wasn't completely blind so that's the reason I didnt notice it.

"Hey just a heads up, but I'm going blind. Like my left eye is pretty much useless. So if you're ever trying to get my attention and you're standing to the side of me, especially on the left, I swear I'm not ignoring you. I literally just can't see you"

Then I'll be sure to crack a couple cheesy blind jokes to put them at ease (ie, if I can't find something "oh my god, you guys, I think... I might be blind")

I now use a cane so it's pretty obvious, but before that I used to say "By the way I'm not wearing sunglasses indoors/and or at night to look cool, I have a blinding eye disease." because I was sick of people pointing out my sunnies, then the awkward convo after where they felt like dickheads when I pointed out why.

I generally try to keep it as simple as possible. Using faces is an okay point of reference, but near/far objects work too.

"If I'm standing right in front of you i can see you ok, but if you're across the room I won't be able to see your facial features in any detail."

That usually works for casual conversation, like if someone notices my cane and asks. At a new job or something important, I'll also explain my other eccentricities (head tilt, twitching eyes). I could pass as sighted and if I'm not upfront about my vision people might think I'm a weirdo (or worse) when they notice that kinda thing.

I used to not tell people and they just thought I was incredibly clumsy, but then one day at work one of my coworkers asked how many DUI's I had since I obviously didn't drive/have a driver's license. Now I just bring it up the first time I do something because of my vision loss. I'll just be "sorry I can't see that..." And then tell them about it. Usually they are fascinated by the story and ask plenty of questions. And once one person in the group knows they all know

I have thick-ass glasses, so it's obvious. I tell people online, especially when gaming. It's usually epically hilarious, like when a friend is trying to explain it haha, such as the last 40 seconds of $1 video. Priceless.