Here is a guide on how to get a Discord User's Unique ID:

1. First you need to enable Developer mode. Go to 'Settings' at the bottom left of your Discord client, like here:

2. Next you need to locate the tab called "Appearance" to the left:

3. You need to locate developer mode and enable it. You can find this if you scroll down to the bottom on the appearance tab:

From now on, whenever you right click a user's name on discord you will get the option to "Copy ID"