It's easy to remove the protective paper backing from vitreous tiles.
Place the tiles in a sink and cover with water. You may add a drop of dishwashing liquid if you like but it is not necessary.
Let the tiles soak for 10 minutes or more. When ready you can lift the tiles and paper and gently wipe the tiles off the paper. Put the paper aside. Swish the tiles around in the water to remove any excess glue and remove the tiles from the water. Place the tiles on a towel to dry.
Once completely dry the tiles are ready to use. If you find some are stuck together just wet them and they will come apart. If they feel a little slimy they still have some glue on them. You may use them like this, they don't need to be completely clean on top as this will be removed during and after the grouting process.