Creating a project in scala is very similar to creating one in java. Here is what the entry class should look like:

package com.example.myplugin; //{$TopLevelDomain}.{$Domain}.{$PluginName}

import org.bukkit.plugin.java.JavaPlugin
import org.bukkit.command.CommandSender
import org.bukkit.command.Command

class PluginName extends JavaPlugin {

  override def onEnable() {

  }

  override def onDisable() {

  }

  override def onCommand(sender: CommandSender, cmd: Command, label: String, args: Array[String]): Boolean = {

    false
  }

}

First, make sure you have installed the latest Scala version located here: https://www.scala-lang.org/download/

Next, you'll want to download Scala Eclipse, available here: http://scala-ide.org/ and extract the download to a folder of your choice.

Once these are both installed, simply open Scala Eclipse.

Lastly, in order for your plugin to work - you need to have some sort of runtime plugin to load the scala library for you, I use this one: https://dev.bukkit.org/projects/scala-loader (place this jar in your plugins folder just like any other plugin)

From here on out, the process is almost identical to java:

1. Press Alt+Shift+N -> click Scala Project
2. Right-click on your project - click Properties
3. Click Java Build Path, then click on the Libraries tab
4. Click Add External Jars, and select your spigot-api jar file
5. Click Apply and then OK

For the project setup, you'll want to create a package so:

Right-click on project -> New -> Package

Name it how you'd like, typically: com.yourdomain.pluginname

Inside of this package, create a Scala Class and name it how you'd like, typically: PluginName

Make the class extends JavaPlugin and override the provided functions for a basic setup as shown above.

Lastly, Right-Click on the folder called "src" and select New File. Name the file plugin.yml (NOT the name of your plugin, but explicitly plugin.yml) and open it.

A basic implementation should look like this:

name: PluginName
main: com.example.pluginname.PluginName
version: 0.1

And there you have it! After you're done writing your plugin, click File -> Export -> Java -> Jar file -> Select your project and specify your server's pluginfolder as the destination -> click Finish

Typically you can simply reload your server to see the changes after export, however some plugins will break on reload, so be careful! I advise always restarting the server unless you know that reloading will not break other plugins.