1. Open the "Minecraft: Java Edition" launcher, but don't press "Play." Instead, select "Skins" from the top menu.

2. Tap the plus "+" sign to add a new skin.
3. On the "Add new skin" page, select "Browse." Find and select your image, making sure it's the specified dimensions and PNG format, and click "Open."

4. Name your skin if you'd like, choose the "Classic" size or "Slim" size, then click "Save & Use" in the bottom-right corner.
When you launch the game, your character will be wearing the new skin.