Heading to the Mecca next week. Never been. Any tips will be appreciated.
Thanks

Check Park's Fly Shop's website for fishing reports. I haven't looked recently, but they usually have good ones. Or just call them and ask.

It's a big place and the best fishing moves around as during the summer. So, checking current reports can help narrow down the where to fish while you are there.

The above advice is good. I spent the last week of September in Yellowstone/Grand Teton back in 2018. When we first got there we stopped in at a fly shop to buy a license, some flies, and ask for a little info. The guy behind the counter explained that the end of September can be a weird transition period. Summer fisheries like Slough Creek have been hit hard all summer and are cooling off, leading to a tough bite, but winter fisheries like the Firehole haven't picked up yet. We ended up completely changing our plan and fished a lake for grayling (I forget the name, sorry) as well as a cooler portion of the Firehole. We also burned a day fishing Yellowstone Lake, on the hunt for some lake trout from shore, but didn't have any luck.
All that to say: ask around at the fly shops. They'll know the conditions and can point you to some overlooked options. I'm jealous of your trip; the colors will be changing and temps dropping. That part of the world is beautiful. Wish I could get up there every year.

Bear Tooth Mountains

beautiful arctic lakes

i wasted a day on the Clark Fork
in Montana
you really need to know what you are doing
technique is key

I have caught cutthroat in Yellowstone lake using a black wolly bugger, but it has seen years ago. .