#morti's methods

You can make this effect in Clip Studio Paint as well, but I prefer to do post production touch-ups in Photoshop. Make sure to fully merge the main subjects of your drawing in a single layer like so. Then make a copy of it

Play around a bit with the curves. Don't make the values too light or too dark. Make sure to find some balance and that it has that backwards Z shape on the graph

Decide whichever opacity level fits the artwork and you should have that subtle glowy/bloom effect. Then you're done :3

Finally, here are the before and after shots. You'll notice that the second one is more atmospheric compared to the first