Applying a custom skin in Minecraft Java Edition
Check the edition, texture file and model before importing.
Confirm the edition
Java and Bedrock use different interfaces. Start with Java Edition in the launcher rather than following a mobile tutorial. Both editions can run on a computer, so the device alone does not identify the workflow.
Prepare the texture
Select the downloaded skin PNG, not a screenshot of the 3D preview. Verify its dimensions and layout in a suitable viewer or editor. Keep the original so you can retry without repainting.
Use the launcher skin area
The official guide describes adding a skin through the Java launcher skin interface. Select the file and the appropriate Classic or Slim model, then save and use it. Check the linked official guide if your launcher labels differ.
Inspect the arms too
Look at sleeve edges as well as the front. A mismatch between model and texture may appear mainly on the arms. A broadly scrambled image also warrants checking whether you selected a texture rather than a preview.
Change one thing at a time
Keep the file constant while checking the model, or try a known working texture with the same settings. Changing everything hides the cause. This explains the workflow without claiming tests on every launcher version.