This is a job that can take a few days (only for the drying mostly) and if you want to get the right results (but it's really quite easy to do). I absolutely loved doing it.

Step 1: Prepare Tiles/Working Environment and Do a Base Coat

1. Ensure you cover the floor and all areas appropriately with plastic (use painter's tape to stick it down where needed). You don't want this paint to get anywhere else.

2. Clean all tiles with a soft damp sponge ensuring all residue/dust is completely gone. Dry properly. You don't need to sand the tiles to make them rough. You also don't need a primer. This only works well if you use 'Histor' floor tile paint (I was recommended to use the floor tile paint on the walls because it's more durable). Definitely a great option.

3. Apply a first coat using either a paintbrush or roller. I initially used a paint brush for the first coat and then later on used a roller to make things easier. Make sure to avoid too much paint on the tiles so squeeze out the brush or roller all the time. Otherwise you'll get horrible looking streaks which you don't want.

4. Leave the first coat to dry for around 6 - 7 hours...you need to ensure the tiles are bone dry before applying the next layer (very important...).

Step 2: Apply Layers

Depending on the darkness/colour of the tile underneath you may need to apply more than one layer. In fact, my tiles were an unsightly green and I had to do around 4 - 5 layers to ensure proper coverage.
Ensure you paint between the grout if need be/if you want to (I used a finer brush for this but you can also use a grout stick). Painting the grout is the tricky part because of the ridge so you need to go over it a few times to ensure it keeps the colour and that this part doesn't get streaky.
Apply as many layers as needed, leaving a good 6 - 7 hours between each layer (as mentioned - very important).
Once complete, use turpentine to remove any paint splatters on the floor or on anything else.

And that's it! I've never been more impressed with how quick and easy painting old wall tiles can be. With the right type of paint you can work wonders. On the photos you'll see that I still need to do one more coat to ensure a smoother tile throughout, but you get the idea. My toilet now looks completely revamped!

Good luck!