This wiki has been updated for the Dimensions 3 update.
To install dimensions the only thing you need to do is as simple as dragging the Dimensions.jar you downloaded into the plugins folder inside your Minecraft server files.
After you have done that, restart your server and the plugin will be ready for use.
To create a portal, you need to go to https://astaspasta.alwaysdata.net/ and click on the Portal creator. From there, you can fully customize your portal.
Once you have finished, go to the "Config" tab and click Download portal file. Now you need to put that .yml file you just downloaded in the ./plugins/Dimensions/Portals/ folder. Restart your server again, and you are ready to go.