There are 2 ways:
By providing them with a custom banner/text link on your affiliate page.
Let's say you provide a custom text link. Here's how to do it.
Go inside your product settings >> Affiliate Program tab >> Use Affiliate Program >> Yes >> scroll down to Affiliate Promo Tools >> Text Link:
Click "Add New Text Link" and you'll see this:
Remember to click "Save".
When affiliates are on your affiliate page, they'll see this text link with their affiliate link that takes them to the custom url.
By manually providing them with a link with the following format:
affusername = affiliate username
sellerusername = seller username
sku = product sku (found inside your product settings >> general settings >> top left)
http://yourcustompage.com/blabla = your custom page