To make use of the Instant Experiences format (previously known as Canvas), you'll need to get to the Creatives step fo the Campaign Creation Flow.
From there, you'll see an option to decide where your audience is going to go when they click on your ad:
Selecting Instant Experiences will give you two options:
If you have created any Canvas on the page you're using, you can select them from the drop down list on the left:
If you decide to create a new Instant Experience, it will open a new window with the Facebook Canvas Editor, similarly to the Lead Ads creation:
Once you're there, simply follow the steps Facebook has designed!
After that, the rest of the Campaign Creation Flow is the same as usual.