I'm an experienced amateur and make no claim to doing this the best way. I find a solid ceramic tile enclosure around the tub and shower to be one of the easier installs. The method I'm most confident with is to lay the grab bar in the position where the tub/shower user will find it most useful and determine which tile squares to drill into. I put masking tape over those squares where I plan to drill holes, put the grab bar back in place on top of the tape and use a Sharpie to make pin point dot at the center of the 3 holes at each end of the bar that I plan to use. I then use spring loaded center punch to put a slight ding in the ceramic right underneath the Sharpie "dots". I want the drill bit to start cutting in that slight indentation.
I use variable speed drill with a 5/8" carbide tile bit (purchased at Home Depot) to slowly drill a quarter inch or so into the tile surface at each of the black marks. The purpose of the masking tape is to keep the bit from wandering. If those holes are more than an 1/8" off it's going to become a messy installation. I prefer to avoid the grouted areas between the tiles and drill into solid ceramic tile (not too near the edges). I'll be using 5/8" Toggler Alligator anchors which I find very versalile and easy to use. At this point I've drilled through the ceramic tile deeply enoughto reduce the risk of cracking the tile and I'll now use a 5/8" SDS hammer drill bit to complete drilling the hole to the proper depth for the anchor. I could have used the ceramic bit in the standard drill to do several inches in but I'm trying to preserve the bit and save time by switching to a hammer drill. And if I started immediately with the hammer drill, I'm more like to crack the tile and wander off the marked hole. If I take my time I can tell if the SDS bit is running into a stud (that's a good thing, right!) Ideally I'd put all the grab bar screws into studs - but I'm not much good at accurately finding studs behind ceramic tile and I have no idea of exactly what's behind that tile, especially on an exterior wall. If there's an air space back there, the alligators will poke into it and spread their "wings". If I get through the tile and my bit is still finding resistance, I'll poke a nail into the hole and if it hits a stud, I stop drilling and a pan head screw without the Alligator anchor in that particular hole. If I go into a stud with a 5/8" bit too deeply, I can still use the Alligator achor - no problem. How deep to hammer drill if you don't hit an obstruction?
Just a bit deeper than the length of your Alligator anchors. BTW, these are made of different material than ordinary plastic anchors and I do NOT recommend those.
I used to use toggle bolts but it required a half inch hole in the ceramic for each screw and I never knew what length of bolt to push through the hole. If the ceramic is solid and sound I'm thinking the grab bar anchors will last as long as the tile is there. Good luck!
PS I recommend a fresh SDS bit in a hammer drill as well as a slow drilling of no more than 1/2"
deep pilot holes. I don't use water to cool the bits. I use slow, steady pressure with sharp bits and a gentle touch with the hammer drill as well.
It's all about not breaking up the ceramic tile and getting a tight hole for the fasteners.