One little tip that people can use, on the last step:

"When I'm finished, in this case I hide the Ink layer since it blends better without it. Then I select the level layer, and use the 'magic wand' tool to select the area that I erased for my tile in the first step. Its a lot easier (espescially for these middle pieces) than trying to select the exact 150x150 square using the selection box tool."

If you're using Photoshop you can select the Marquee tool, and then in the Options bar up top choose the "Fixed Size" option. Set the size to be 150 px and 150 px. Now, everytime you use the marquee tool it will always be exactly 150x150.

Or you could use the Magic Wand. . . .