We only have experence with Tlauncher skins upload, but do not have enough knolwdge on SKLauncher if that is what you wish to use, sorry.

If you want to get a new Minecraft skin, visit THIS website, and pick your skin, and download it. Next, visit https://tlauncher.org. Once on Tlauncher, select the button "Download Tlauncher" (Unless you have already). Next, create an account on Tlauncher (Unless you have one already), and there will be a user space once you have signed in. Your screen should look like the picture below. For skins, select the button "select skin" and choose your downloaded file. Open Tlauncher, and when activated, make sure that looks like the two images below. Click on the account button to the bottom left. Then select create a new account, and log in to the same account as the one you made in Tlauncher. Then click on save an account. Once the account is saved, select the version 1.21.4 down the bottom of your screen, select the button "play the game" and wait for installation. When the installation is complete, Tlauncher will close down, and start up Minecraft. When Minecraft shows up, there will be three main middle buttons. Select the middle button named multiplayer, then at the bottom of your window select the button that says "direct connection" (if you just want to visit, and don't want to stay, or select "add server", and add the name of the server (creeper_12ant's Minecraft Server), and add the ip address: creeper12ant.online, and join the server.) and insert the ip address as follows: creeper12ant.online, then select "join server".