Took me a long time to get a locator stop e thing, I probably cleared a dozen villages and twice that many plains biomes before finding one (and shortly after, a second one).

I found the alter while sailing past before even finding a runestone for him, probably just a lucky break.

if memory serves i think i've seen most of them in the stand alone stone towers that are guarded by fulings. But i found the altars before the stones as well. big giant 'finger' stones in a circle, hard to miss if you get near one.

I did not find one until way after I had found his summon area. The summon area is like the size of a skyscraper in a football field. The where to find that stones are like the size of a furling and have maybe one or two 'shaped' or 'worked' rocks around them. Just go looking and you will stumble over one or the other, but the summon alter is the easier one to see -- if you get near it, you will not miss it.

I had the same "problem" i found the boss altar but i never found the vegvisir for it LOL

Same here , stumbled upon the alter while out sailing, it's hard to miss.

Iirc it was in the fulling forts, not villages.

But it was forever ago, could be wrong.

Plug your world's seed into the Valheim World Generator to see where to find Yagluth, or anything else you're having trouble finding...

https://valheim-map.world/

It's completely random, so there aren't really any tips that anyone can provide, other than to either keep combing every plains biome you see, or to plug your seed and into the online map generator and use that to find a vegvisir. Yagluth's vegvisir is not a great deal more common than his actual altar, IME.