Login to your http://www.godaddy.com account.
Go to My Products > Domains > Manage
Click on DNS next to your preferred custom URL.
Click on Edit sign, next to the CNAME (If you want your main domain i.e www.mycustomdomain.com to point to the Learnyst account.
Keep the host name as www and fill out the Points to value as pro-ssl.learnyst.com and save the changes
Alternatively if you want to create a new sub-domain for your url, you can scroll down a bit more and click on ADD.
On the next prompt, select CNAME from the dropdown and you will see a screen like the following
As the Host, input the subdomain prefix name ('courses' if your desired subdomain is courses.mycustomdomain.com) and add the Points to value as pro-ssl.learnyst.com
If you have configured your main domain (www.mycustomdomain.com for example) to your Learnyst account, you need to add a redirection as well so that mycustomdomain.com redirects to the same page. To do so,
Scroll down a little more until you see the Forwarding on the right hand side. Click on Add
In the forward to section select https:// if you have added pro-ssl.learnyst.com as the CNAME host previously. Set the forward type as Permanent 301 and select forward only and save the changes.