So, I'll be using two examples over here. Before that, let me explain a few points.

× To choose your base for a face swap, I would highly recommend deviantart. You get the best cosplays over there for free. Pinterest is also a good option, but you never know which you can use and which you cannot.

× Model (used in the face swap) vs base (used for face swap) 

The left side is the base and the right side is the model.

Front viewed face = front viewed face / anything if the body proportions are still right.
Slanted face = front face / slanted
Side viewed face = Side viewed face
Looking down = literally anything since you're going to chop the base's head.
Looking down side = looking down side

Now, let's begin.

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now, as you can notice, the colours don't match at all. Well, let's make them match.

1) Remove the base's face. Literally literally warp is or do whatever you want.

Don't feel sorry for it. Never. (just kidding be a little decent). I want the background to be intact so I used the liquify tool to warp it.

2) Now, you see how the base's face is tilted yet I'm going for a front viewed taehyung? Well, it depends on the neck placement. You see, the neck plays a major role in fitting your face claim with the base. Here, we're not going to use the base's neck but rather taehyung's neck.

Once you're done placing it like this, use the smudge tool and brush down taehyung's neck (basically his skin colour). If we use the base's neck here, then it would look really out of place.

Now, remember a small thing. The base's face and the face claim's face should be of similar size. Which means the body proportions should not be out of hand.

3) Adjust the brightness, contrast of taehyung.

Adjust the colour balance.

And then adjust the colour balance of the base background.

4) Once you're done, using the airbrush eraser, slightly erase parts of taehyung's neck to bring in the base's neck. Once you're done, bring all you've done to a completely separate layer and adjust the colour balance.

(when your edit begins to look like the PicsArt ones)

Eh well, this is just the base of the face swap. You'll definitely have to add in more elements for the edit to be complete. (which I'll try to do soon)

Why tf is it so dark oml. I feel like i should have just shown the final one 😭

Once again, this is just the base of the face swap you'll be doing. You'll definitely have to work on it for the final outcome. I'm just too lazy to do it

Do lemme know if you have any doubts!