To make modals work, you start with adding a "Bootstrap Modal" block. The first thing you need to enter is
Modal ID
This is an ID element added to the modal. It can be something like "login-modal" or "popup-modal", as long as you know this ID hasn't been used on other elements.
Great, now that this is entered, you can get to the content part. Bootstrap Modals are populated using Stacks. So be sure to add a stack for the modal you want to create first. Stacks can have multiple blocks in them, so that makes it possible to have a very extensive modal.
Body/Content
Select the stack (or multiple stacks) you want to show up within the modal. You can also drag/drop the selected stacks, to order them. The order of selection will determine which stack will output first, so be sure to have them in the correct order if you have multiple selected.
A modal can have a header, showing a title and/or closing button.
Show header
A yes/no value, giving you the option to leave out the header by selecting "No".
Modal title
The title being shown within the header.
Show close button
A yes/no value, giving you the option to leave out the close button by selecting "No" (normally positioned in the top-right of the modal).
A modal can also have a footer, perhaps showing some buttons or some text about the contents displayed within the body of the modal.
Show footer
A yes/no value, giving you the option to leave out the footer by selecting "No"
Footer
Select the stack (or multiple stacks) you want to show up within the footer. You can also drag/drop the selected stacks, to order them. The order of selection will determine which stack will output first, so be sure to have them in the correct order if you have multiple selected.
IS THAT IT?
No. Now you've entered all this, you can save this block. But this will only mean you have added a modal, but you need a button to trigger the modal for opening it. So you also add a "Bootstrap Button" somewhere on the page where you would like. On the "Basics" tab of this block, you select "Modal" within the "Element" field. You will see a "Modal Target" field show up now. In here, you enter the value of the field "Modal ID" (this is the very first thing you enter(ed) for the "Bootstrap Modal" block, at the top of this page). Also add a value within the "Text" field at the top of this block, something like "Open the modal" and save the block. You will not see a button appear. Save the complete page and click the button! It should open op your modal now.
In case your modal won't appear, add your "Bootstrap Modal" block near the end of the page. It could be that a theme uses different classes, where it will block/hide your modal.