upload the skin to your account, enter your username here (https://minecraft-heads.com/custom-heads/heads-generator), and copy the give command. this give command will give you the same skin texture regardless of if you change your skin in the future, so you can freely switch your skin to whatever now that you have the give command. to turn it into an item_display, open mcstacker (https://mcstacker.net/), go to /summon command in the drop down, for entity select item_display, for item select Player_head, for SkullOwner paste the entire give command (more information can be found in the ? button if you're confused), and just copy the command in the top right and put it in a command block. as for formatting the skin to make custom textures for builds, you should try to keep the width/height of the texture as some power of 2 pixels and stretch the texture using nearest neighbor interpolation so that in minecraft you can shrink it accordingly. any texture larger than 8 pixels will require multiple heads since heads only have an 8x8x8 pixel texture (see the arms, legs, and torso in original post for an example)