15 Puzzle
The 15 puzzle is one of the most popular sliding puzzles, containing 15 squares that can be slid around in a 4x4 square grid. These squares usually contain numbers, as shown above, however there are also variations that have images on them instead.
This method works for any size of sliding puzzle, not just the 15 puzzle specifically.
Focus on one square at a time. You can do this step intuitively.
Place them similarly to the previous step. Most of the 15 puzzle is just figuring out what order and where to put certain squares.
This can be done in the exact same way as the first row.
This step is where it starts to get difficult. You need to pair two squares at a time. If you're pairing them on the top, they have to be in the opposite order. If you're pairing them on the bottom, they have to be in the correct order. Then, just move them both at the same time.
You can solve this with enough thinking. In fact, you don't need thinking. As long as you don't interfere with any already solved squares, you can just slide them randomly and suddenly it's solved.