Well, setting expectations first: they're unlikely to give you complete light blocking because that takes multiple layers. You would want to have blinds mounted inside the window frame, and then a layer of curtains around the window.

And if your windows face the sun you'll of course be fighting more than if your windows don't face the sun.

So: yeah there are limitations since you can't (or won't be able to) install blinds, and can't (or won't be able to) change which room you are in and which direction the windows face.

Overall guidance is to eliminate the gaps.

You will get less light bleed if you can use some small thumbtacks or pushpins and push them through the edges of the fabric and into the wall surrounding the curtains. Basically stapling the fabric to the walls.

On the top and bottom you try a couple of things.

On the top you can drape extra fabric across the gap - like a valence.

On the bottom you can lean something across the bottom of the fabric to make the curtains close the gap. An extra pillow for example.

Finally: invest in a comfortable eye mask.

The better eye masks are extremely comfortable and have foam surrounding the "eye wells" and they won't press on your eyelids and eyelashes.