Click Customization in the left menu, then the Profile tab.
Under Links, click Add link.
Enter a title and the URL. Repeat for more links.
Click Publish.
Where your links show
You can add up to 14 links. The first link is shown on your channel header next to your description. The others appear when someone opens your channel's "more" section (YouTube Help). Put your link in bio page or your most important link first.
Do not forget video descriptions
Channel links reach people who visit your channel. Most viewers never do. Add your link to the first two lines of every video description, before the "more" cut, and pin a comment with it.