The easiest way to make a tile is to colour-print it on paper and glue it to foam core, card stock, or cardboard (non-corrugated).
Foam core is ideal, but slightly costlier. Still, you can get a 6' by 10' sheet for a reasonable amount at an art supply store.
Card (stock or board) is cheaper and easier to find. The gotcha with this process is that the glue shrinking will cause the tile to bow along the grain of the card. Fortunately there is an easy fix that also enhances the durability of the tile: cut two pieces, turn them 90 degrees, and glue them together before attaching the printed paper. The turn-and-laminate trick ensures that the grain of the card is aligned at right angles, preventing either half from warping. As a bonus, the base is stronger than a single piece.
For all these purposes, the ideal glue is just an ordinary glue stick (acid-free if you intend to have the tiles last for years).
I've used all of these materials. Card stock is the cheapest and easiest to work with, but results in thin tiles even if you laminate. Card stock also gives you the option of skipping the paper and printing right into the card, if you don't care about durability but want something with more heft than paper. It warps very little or not at all from just the moisture of an inkjet printer.
Card board is great, but takes that extra work for quality and durability. Foam core is best, but harder to get and needs a very sharp knife to prevent ragged edges. The advantage of foam core is not having to laminate; the advantage of laminated card board is the resulting tiles have a satisfying bit of weight and solidity to them. In any case, invest in a decent cutting mat and don't skimp on the scalpel.
As a bonus, if you're set up for making tiles with any of these methods, you can easily get into 3D papercraft props to add some verticality to your layouts, and there are many, many papercraft PDFs available online for free and sale.
