I have a couple of 4"x4" ceramic tiles that are cut just slightly too wide. I need to take off about an eighth of an inch. I expect that's too little for the standard score-and-snap approach. What's the easiest way, given that I don't have any tile cutting tools.
The best approach is a tile saw that can make small precise cuts along the length of a tile. It is a power tool like a table saw but uses a diamond impregnated blade and a water cooling system. They can be bought or rented.
A cruder approach if the tiles are not too big (4x4 should not be a problem)and there are not too many tiles to cut is a tile nipper.
These chip off little bits and you can work along an edge to do the whole side. However, some tiles are especially brittle and some are prone to cracking. I would not try it unless you have some spares to test on and in case the selected ones break.
And, as noted in Jimmy James' comment, eye protection (e.g. safety goggles) is a must for nippers and also for the saw. A random bit of ceramic shard or grit shot into the eye is no joke.
Images and links are illustrative only, not an endorsement of goods or sources.
I own a pair of tile nippers - I bought them first, they were cheap. Unfortunately, they didn't work (pretty much "at all") on the tiles I was using - breaks would be more or less random.
So I bought one of these dry diamond blades for an angle grinder - if you don't own an angle grinder, this might just be one of the few times I'd say that the cheapest knock-off you can find with a warranty that's slightly longer than your project will take would be appropriate - personally, I had one already. Eye protection, dust mask, work outside or somewhere with massive dust collection. As compared to the wet tile saw, no built in fence, (I was able to set up wood to clamp tiles and act as a fence when needed) not wet so there's dust, much less expensive. As compared to the nippers, no comparison, this just works. I was even able to miter-cut some tile to make up for not being able to find bull-nose for 50 year old tile I was reusing.
Image is an example, not an endorsement. A dry or dry/wet blade is what you want for dry cutting - a wet (only) diamond blade will not be happy if used dry.
However if you literally have only a couple of tiles you need trimmed, bringing them to a tile shop might be the best option - pay them a couple of bucks to trim them for you. Or the cheapest (if a bit slow) tool would probably be one of these silicon carbide rubbing blocks:
which you could just grind down the edge with.
Use the “wet saw” for free at the place you bought the tiles. The store will usually include one cut per item bought for no charge. That’s what I’ve done (at one of the big-box stores). If you can’t get that where you bought, take them to the store which does have such a saw and pay 10¢ – 20¢ per cut.
There are hand saws sold for awkward cuts in tiles Here's one from B&Q (UK). I've got one and have used it for just the sort of job you describe. I suggest scoring the glaze first, then running over the scored line with a marker pen before cutting, so your cut line is both clean and clear to follow.
It's not really a saw blade, more a grit-coated wire of around 1mm diameter (so 1/3 of the amount you want to take off is the with of the cut).
This example may be a bit too fine but you can also get tile files which allow you to take off even less. Again I have one and it wouldn't be too much work to do what you want with that.