You will probably need to remove some old tiles first
Also, work away from yourself as much as possible, and be especially careful near a window. Old adhesive can be removed as a separate step in the process. The less damage you do to the underlying wall the easier it will be to get a good result with the new tiles. Rather than start in a corner it is better to begin in the middles, say the middle of a window. Use your tape measure and mark the middle point. Then using a tile, you can mark out where each will go along a line to the corner to check that tiles will fit without too much adjusting in the corner. You can start with your first tile placed either with its middle or edge on your first mark. Apply adhesive to the wall with your trowel at about 60 degrees to get an even thickness, or you may find it easier to put adhesive on the tiles, one by one, as you go. Place your spaces between the tiles as you work. Cutting tiles is done with a tile-cutter and may need a little practice to master, so hopefully you have allowed for wastage of a few tiles. To cut out shapes it is best to make a paper template and transfer the shape to the tile, and then use the wet cutter. It is best to leave tiles for several hours or overnight before grouting. You may need to colour the grout before using. Any spacers on an edge may need to be removed, before you push grout into the joints over the whole are of tiles. It is important not to let the excess grout stay on the face of the tiles for too long. With traditional grouts you have about an hour, but with some epoxy ones, only about ten minutes. The grout excess is removed with a damp sponge, wringing it out in a bucket of clean water. The internal corners where tiles meet are usually finished with a silicone sealant or mastic to make them waterproof and give a neat appearance. It is worth using masking tape on the tiles first, leaving the width of join that you aster happy with. Then, simply run a bead of sealant along the length of the join, keeping an even pressure and speed. Run along a wet finger to give a shine. Once set the masking tape can be removed and the tiles polished over to remove any haze.