I don't do this often enough to remember this so i rather document this incase I have to do it again. If it helps me, maybe it helps you as well.

Create a new VM

The first step is to create a new VM. There are way to many Hypervisors and i'm not gonna explain each one of them here. I'm going to assume you already possess the knowledge to create one from scratch or from a template existing in your environment. 
The VM can be very basic, but the important thing to take a note of is the MAC address generated by the Hypervisor. You will need this address later for Citrix

Adding a VM to the Pool

1. Login to your Citrix Provisioning server and start the Citrix Provisioning Console
2. Go to Sites > Device Collection
3. Right-click on the device collection you want to add a new machine to and select Create Device
4. Enter the following information:
   • Name - Give it the same name as your VM, most of the time it's a logical name based on your environment's naming scheme
   • Description - Enter whatever you feel is best here or leave it empty
   • Type - Select the correct type here, it all depends if you use the new machine for testing (acceptance), maintenance or production.  This is required if you use the UPDATER method of deploying/updating images
   • Boot from - Leave this at vDisk, most of the time the machines use vDisk
   • MAC - When you created the VM the hypervisor generated this, you can find this in configuration of the VM under network setttings
   • Port - by default this is 6901 and be left as such unless your environment uses different ports
   • Class - by default this is empty, unless your environent uses this
5. Click on Ok to add the new machine
6. Right-click the newely created machine > Active Directory > Create Machine Account
7. Set the domain to your Domain if not set and select the Organisation Unit you want the machine to reside in. Then click on Create Account to create the machine in Active directory
8. Right-Click the newely created machine again > Properties
9. Click on the vDisk tab
10. Click on Add and add the vDisk for the catalog, this will ensure the new machine boots up the correct image. Click Ok when you are done
11. Log in to your Citrix Delivery Controller and start Citrix Studio
12. Go to Machine Catalogs > Right-click on the catalog you want to add the machine to > Add Machines
13. Enter the DNS or IP address of the Citrix Provisioning Server and click on Connect
14. If should now list the Site tree, expand it and put a checkmark in front of the catalog you want to add the machine to and click Next
15. The list should show you the new machine you've added on the Provisioning server, select it or multiple and click on Next to finish adding.
16. Boot up the created VM"s and check if they boot into the correct image
17. Back to Citrix Studio, go to Machine Catalogs > Right-Click on the catalog you've added the machine to > View Machines
18. The list should contain your newely added machines and the registration state column should say they are registered now. 
    Congratulations, you've expanded your VDI pool with a new machine! 
    If the registration state keeps saying unregistered, most likely the VM doesn't have a network connection to the Citrix Delivery Controller, maybe it's in a wrong VLAN, DHCP doesn't work, could be anything really....