When I bought my boat it was out of the water so I never saw it rigged until I launched it.

There were two sail covers and I used the older one because the newer one had a zip which had rotted.
I now realise the older one was a stack pack with two rods to insert. I had put the rods on a shelf with other stuff over a year ago and forgot about them.
The lazy jack lines I just tied off because I couldn't figure out where to attach them as there were no slide inserts on the boom.

I've had a new zip sewn in and all the stitching re-done, so I decided to fit the stack pack and lazy jacks. I spent all day yesterday fiddling with the lazy jack lines and making adjustments.
I ran the mainsail up while on the mooring to check it all out, but I'm not sure what to do with the stack pack when under sail.

Do I slacken off the lazy jacks a bit and let the stack pack/cover flop about?

Or do I tie it around the boom securely?

The mainsail is loose footed and the cover sits on top of the boom.

Just slacken it off. Mine has three tapes that go round the mast they tend to hold the front up about level with the top of the boom. The rear of the cover attaches to the topping lift so that comes down when the topping lift is eased. The plastic conduit tends to keep it tidy. Here is the video I made 6 months ago when I was trying to demonstrate how the lazyjacks don't effect the sail.