I'm not the faker, but here's the gist of it.
Open the current banner (at the time) in Photoshop. Select which characters you want to add in and find pictures of them. Open them in Photoshop too.
Use the lasso tool to outline current characters on the poster that you wish to move, copy and paste those renders onto new layers, and use the paint or smudge tool to add minor details on those characters that may have been hidden before (e.g. Greninja's tongue). None of these details have to be remotely perfect since the final image will be blurry as hell.
Use the smudge tool on the background layer (the original banner image) to fudge the scenery and cover up characters in their old spots. Then copy and paste each of the characters you want to add in from the files you have open. Resize them to fit, and, to make sure no one traces the image back to the reference you used, use the smudge, paint or lasso tool to move portions of the body into different poses. Like, say, Kazooie's wings. Make one go up higher and the other come back across. Now it no longer matches any known render and the blurriness will cover up your mistakes.
Finish the background on the edges with the smudge tool, draw in some overlapping layers with colors eyedropped from the image itself, find renders of the Smash Ultimate logo and Switch logo, paste them as new layers to cover up mistakes, look it over for small missing details, then save it on a USB, walk to Kinkos, print it out.
All of this can be accomplished in an afternoon.
As for the Grinch and store hanging fixture prints, one of two things:
1) The guy actually had access to some of this stuff from this workplace, either through a friend/family member or his own job, and used them to legitimize his leak. OR
2) He looked for a company that does this sort of promotional printing for Nintendo or Bandai Namco and, much like we did, found their video on how they print these things and made that in Photoshop too, printed at Kinkos.
Then, to justify the blurriness he needed, he sent a Snapchat photo with a duplicate account, disguised it as a private conversation gone wrong and used a poorly "hidden" name in the photo to make sure people trying to discredit the leak could find this guy on LinkedIn and find the printing company on his resume to make it all the more real. Pick someone in France with very little on their resume to avoid the possibility of him, say, coming onto Twitter and just saying, "Nah, that's not me." This is the most clever part. The rest isn't really any different from the Palutena tease before Smash 4 or other blurry Photoshop "leaks".