“With Apple’s release of Xcode 7 came an important change for all iOS and Mac developers–free provisioning.”
So all you need is an apple id and to configure your IDE.
There are a lot of guides available out there, so this post is not going to be another one:
– Xamarin Developer Guide
One thing that can be confusing is that first, you need to create a Xcode project with the same “Bundle Identifier” and then download the free provisioning profile. Just pay attention to the uniqueness of your bundle identifier otherwise, it won’t work.