First, open a new file. Let say we start with a 128 x 128 pixels document.
Here, we will go for 9 tiles. So, you need to expend your image by 300%. Image- Image size
Now is the time to setup the grid. First, we want to see the separation between the tiles.
edit - preferences - guides, grids and slices
Gridline evenly: 128 in pixel (the image will be guided at each 128 pixels)
For easier pixel drawing, we will add more guides.
You can also add more subdivision if you find it easier.
Make sure you draw with the pencil tool and that your eraser is in pencil mode.