You can use the FSRI block to manually select posts to display in the Modern Homepage and Modern Sidebar, to display specific posts. However, we've heard feedback that finding the post IDs and building a list can be tedious.
The new Admin > Feast Plugin > Post IDs page makes it simple to create a list of post IDs.
Simply click the button of the Post ID that you want to add, to build out your list of posts:
From there, simply copy and paste into the FSRI block.
Note: this page will list all posts on the site, so sites with 500+ posts may take some time to load.