How do you grout tile?

If it looked fine for a week and then cracked or crumbled, the usual culprits are too much water in the mix or wiping too early. Mix it stiff, like peanut butter. Press it in diagonally across the joints with a rubber float, pack it properly. Wait until it's firmed up a bit (15-30 min depending on the product) before sponging, and wring the sponge out well, a wet sponge pulls grout out of the joint. Wide joints, over about 3mm, want sanded grout. Seal it once it's fully cured.