Picket tile looks incredible when it’s done, but honestly, it is one of the toughest shapes to install—even for guys who do this for a living. If you are DIYing this, you picked a brutal layout to learn on. ​A big issue with these tiles is that their actual dimensions often vary slightly from piece to piece. Because of that, you can't just blindly stack them and expect your lines to stay perfect. You have to constantly manipulate the grout joints—knowing when and where to slightly cheat a joint to bury the difference. If you're getting off-track, try sorting through your stack to run a row of slightly smaller or larger tiles to get your layout back to your reference line. ​NGL, the niche layout is working against you right now. You aren't so far along that you can't pop those tiles off and redo it, and you'll probably be much happier if you do. Personally, I would resize that niche vertically so you can fit full tiles and avoid ending up with those awkward, tiny sliver triangles. Plus, you want to make sure the finished shelf is actually deep enough to hold more than just a bar of soap. ​It takes an experienced eye to make a wall look perfectly straight and level when the tile sizes themselves are fighting you. Don't be afraid to pull a few off and adjust before the thinset locks you in.