1. How to mix mortar. Took a little practice, but at the end I felt like I kinda sorta knew what I was doing.
2. Keeping mortar spacing consistent was essentially impossible for me, as a beginner. Especially with real stone and so all the stones are different. However, I concluded that the variability isn't too noticeable once you take a few steps back.
3. I should have used grout release on the hearth stones before putting them in place. They got stained and I was kind of bummed about it. Have mostly got over it since then
4. I really like working with stone! Suddenly I saw all sorts of places inside and outside at our place that it'd be fun to use stone. Won't end up doing it I'm sure - all of the projects would be complete nice to haves, but it would be really fun and I like the look of stone.
Can't wait to get some use out of the hearth next year!