To add your flow to your Thinkific page, first, create a new page or select the one where you would like it to appear (Design Your Site -> Site pages), then hit "Edit Page".
On the left sidebar, select "Add a Section", then in the pop-up modal choose the "Text and Media" option.
Enter the section, then change to code view.
Add the iframe code of your flow here. About the iframe, you can read more in this article.
Changing the size of your flow
Upon adding your flow to your page you might notice that it appears too small. To fix this, all you need to do is add some extra code to the one above to specify the size.
In Thinkific, unlike other similar sites, you can specify the width of your element with the percentage of the size, however, for the height, you need to use the pixel equivalent. Don't worry, it will not prevent your flow to resize to smaller/bigger screens.
<iframe src="INSERT THE STANDALONE IFRAME LINK OF YOUR FLOW HERE" width="100%" height="800px/"></iframe>