Bedrock
Open Minecraft to the home screen. Under the character model, click on the hanger. When the new window opens, click Choose New Skin. Find the file that you downloaded or exported and double-click it to open it. It will ask you which model type you want to use. Choose classic or the one with the thicker arms. Click confirm and you should be good to go.
Java
Open the Minecraft launcher. Instead of pressing play, go to the skins menu near the top of the screen. Press the New Skin button. Name the skin whatever you like. Set the model to classic. After that, press browse. You need to find the file that you made or downloaded. Double-click the file to open it. Once the skin loads in, press save.
Mobile
Downloading and using a skin is a bit trickier for mobile. If you are using one of my skins, when you hit the download button it should take you to another page that shows only the picture of the skin. Press and hold on this skin then press Add to Photos. Once you've done this, open Minecraft. Click the button under the character model that says Dressing Room. Go to one of the blank files and click Create Character. On the left-hand side, there will be a list of 5 icons. Click the one in the middle that look like a hanger. Press Owned and then Import. Press Choose New Skin then select the picture you just downloaded. It will ask you which model type you want to use. Choose the one with the thicker arms. Click the button in the top left three times to go back to the main screen.