If you have a gap between the areas where you're going to install your tile splashback, such as space between two cupboards, you'll need to create something to support the tiles. Use a stud finder to locate a stud wall or a noggin behind the wall and mark its location. Use your long spirit level and lay it across the gap, this gives you the straight line you need, so that you can butt your timber up against it. Hold the timber against the wall and flush with the spirit level, then drill it into place.
Put drop sheets over the benchtops and floor to protect them from any grout or tile mastic. To make the tile mastic that will glue the tiles to the wall, add a little water to your mixing bucket then add the mastic powder. Put on your dust mask and mix this until it is the consistency of toothpaste. If it's too runny, add more mastic powder, if it's too thick, add a little more water.
After you've mixed your mastic, use your 8mm notch trowel to apply it to the wall. Make sure that the wall is clean and dry before you start. Starting from near your benchtops, apply enough mastic so that you can lay your first row of tiles.
Laying the first row of tiles gives you the foundation to lay the next rows, so it's important to make sure that its level and evenly spaced. Starting at the end away from the corner, lay the first tile, about 1mm above the benchtop but flush to the end. Lay more tiles next to the first, putting yellow spacers under them to make sure they're the same height above the benchtop and spacers between them so that they're evenly spaced apart. One you've laid about half a dozen tiles, use your spirit level to check that they're straight. Repeat this process as you continue to lay more tiles.