This is a little tutorial on how to install a custom map on a server as it is different to installing it on a singleplayer world. It is assuming you already know how to start a server and already have it ready. If this tutorial helps you please leave a diamond!
Step 1: First things first, download the custom map you want to install.

Step 2: Save the map to somewhere that is easy to access, such as your desktop.

Step 3: Drag and drop the world into your server folder.

Step 4: In the server folder, go to the 'server' note document.

Step 5: Go to where it says 'level-name' and type the name of the custom world exactly as it appears.

Step 6: Save the document, and run the server.

Step 7: Now you should spawn at the custom world that you downloaded!

If you have any questions be sure to ask in the comments section. I hope this tutorial helped!