Well I mean, yon9hee you asked, so, who am I to not deliver. 

You need a couple things as a standard. A canvas, multiple renders, and well, any other asset you need, pngs, textures, PSDs, filters, whatever. Best to have them prepared before you begin. 

So my process looks like this. 

First, I set up the position of my renders. Just vaguely is alright, especially if you're using more than two. 

Then I decide what I want to do with them-- have a vision before you start. What I'm going to do here is make half of the graphic the 'devil' side, and the other half of it an 'angel' side. 

So I usually start with the background. Focus on one side at a time.

So I roughly blend some textures together. 

It's okay to be muddy/clumsy or leave blank spots, because it's all going to get covered up by the PNGs anyways. It's also okay to keep the colours all over the place, because I'm going to put a PSD filter over it all at the end. 

Repeat this for the angel side, and done. Make sure to keep your workspace (layers) organised! 

If you're worried about accidentally leaving transparent spots, you can add a plain black/white square under your whole canvas at the end. 

(Sorry for zooming out, I did that while blending and forgot to zoom it back in for the screenshot lmao.)

But anyways, I work on the renders after that, adding shadows and stuff. 

Finally, we can start on the PNGs. 

Basically what I do is literally just... pile them on. Think of it as 'I'm going to throw Yohane into a pile of plushies and make her sit in it for a pretty photo'.

Try to keep the pngs to the same theme-- you start with flowers for this side, you go with flowers. You start with aesthetic lamps, you keep the artstyle similar for the rest of the pngs.

Of course, you don't have to make them match like I do. 

When I do multiple characters (like the whole group of Love Live girls, for example), I like to pile candy on just one character, flowers on another character, fire on another, gold stuff and clocks on another. It all depends on what you want to do with the edit. I just focus on 'characters' because it's easier to keep track of graphic composition like this.  

Anyways, next! You can see there's still plenty of space left. 

Well, here's where you add two things: comic or text-type pngs, and actual text. And watermarks too, if you have any. Try to fill in all of the empty space with these pngs to make it look fuller than it actually is. 

As you can see, I have an empty space in the middle. I did not mean for that to happen, and usually if you're doing more than two characters you probably won't have that empty space haha. 

But I'll see what I can do about it. Maybe add another render, or add more pngs? I can also just leave it blank because it looks nice like that.

There we go.

Of course, the final step is to add PSDs and filters to your liking.

Hope this helped! :)