Right off the bat I’m going to say that if you’re not already using layers in your image / photo editing software, you do need layers to [anywhere near easily] make use of these overlays. That may mean using new software – the GIMP & Chasys Draw IES are free, while apps like PaintShop Pro is starting to go on sale [just bought Ultimate 2023 version for $35], and there are loads of others. More importantly, you have to learn to use them. If that’s more work than you want to tackle, that’s cool – just skip this GOTD, because while it’s possible to use these overlays otherwise, the amount of work more than doubles, while the results are likely to be iffy.
If you haven’t worked with layers yet, think of stacking sheets of paper. Each layer normally has something on it, for example it may have the photo you’re working on after you applied a filter or plugin. At maybe its simplest, each layer holds a step in your editing that you can return to without scrapping everything & starting over. A layer also has different ways that it can blend with the layer beneath it and can have full or variable transparency.
An easy example, there are 100 images meant to look like falling snow on a black background. Checking one image, it’s 4k x 2800 pixels @ 96 dpi. You can either create a new layer, drag or import that snow image to that new layer, sizing it at the same time if necessary, or you can work with the snow image beforehand, enlarging it, perhaps with software that uses AI, then import / drag that to the new layer. That snow pic is all you'll see once you’ve got it on that new layer, so you want to turn the blending mode to If Lighter. Only the parts of the snow image that are lighter than the image on the layer beneath it will show, and you’ve just added falling snow to your photo.
But wait… what if your photo is of a person up close? Falling snow in the background may be cool, but in front of their face, not so much. That’s where / when you add a layer mask, which controls what parts of that layer are blocked, and which show through. In P/Shop Black blocks & White allows, so you’d add the default mask, which is all white, then paint over the person using black. All the snow’s now behind them. [The specifics on how you do this sort of thing vary with the image editing software you use.]
You’d probably want to use layer masks the same way with the other included overlays too, especially the Vintage Look & Film Light Leaks collections, but here you might use a bit of a twist rather than painting over someone(s) with a brush. While one color blocks & the other allows, a gradient allows all sorts of values, graduate steps, in between. A radial gradient for example will keep any effect from showing over a person or persons [you can certainly have more than one gradient], while allowing the overlay to gradually fade in in the background. Now change the layer’s blending mode, and you can have a quite nice effect, looking nothing at all like the overlay you added.