Essentially all you need to do is write on a porcelain object with the porcelain pen and bake at the temperature indicated on the pen (temps and times vary with different pen brands). The Dollar Tree has a huge selection of plain porcelain dishes for $1, if you need some. If you have an oopsie, its generally easy to wipe off the paint pen with a damp paper towel before it completely dries, within a minute or so. After that it might be harder to remove, and then after it is baked it is on for good.
If you're lacking inspiration on what to write on them, I've got some ideas:
Phrases that go along with the item being written on. For example, "Keep Calm and Snuggle" on a mug.
If you still need more inspiration, I've gathered a few of my favorites from online. All of these are black on white, but don't be afraid to use multiple colors of porcelain pen or write on a colored porcelain object!