Putting grout over uneven tiling -- often found in mosaic tile patterns or other similar tile types -- is essentially the same process used in grouting normal tile, with one basic difference: Instead of passing the grout float over the tile once and being done, you will likely have to make repeated passes over the tile and perhaps even use your finger to cram the grout in between the uneven tiles. It's not especially difficult, but it will be time consuming.
Mix a batch of grout in your bucket. Make sure you follow the instructions on the bag carefully, as deviating from them can cause your grout to be of inferior quality, which will cause problems down the line.
Clean off the tile and the lines between the tiles of any thinset mortar, grime or other debris. It is important that the grout be applied to a clean surface, otherwise it might not set properly. If your grout mixture package suggests you spray down the tiles with water before applying grout, do so now.
Use your spatula to scoop out a large glob of grout and place it on the tile.
Hold your grout plane at about a 30 degree angle and use it to spread the grout into the cracks between the tiles.
Use your fingers to press the grout into any spaces the grout plane did not fill with grout.
Repeat steps 3 through 6 until you have applied grout to all the tile.
Allow the grout to dry for the amount of time recommended on the packaging -- typically about 10 minutes -- then wet your grout sponge and use it to wipe away the excess grout from the face of the tiles. This will require several passes, thanks to the uneven nature of the tiles.