first of all you will need the following programs until we can begin:
Gimp: https://www.gimp.org/
Gimp vtf plugin: https://developer.valvesoftware.com/wiki/GIMP
vtfedit: http://nemesis.thewavelength.net/index.php?c=178
GCFScape: http://nemesis.thewavelength.net/?p=26
sfm or gmod: *see steam store*

so this part of the guide will mostly be in detail over gmod as i am inexperienced with sfm.
so of course the first thing you want to do is create your image, simply pose your character in the way you want it to or use the animated prop tool, now depending on the team color you wish to use i advice you use different color. for team red you use a blue screen as there are no blue colors in red teams cosmetics or other models. in case of blu team i advice you use a dark red color as the background as there are also no red colors in blu teams colors. then you take the screenshot and we can go on to the next step.
if you use gmod i advice you use this addon if using gmod: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=104635769

now you have the main image. now you have to edit the file to properly use it as a mod. so first you put the original file into gimp, it is located in tf2_textures_dir.vpk and then in materials\console\characters
when you did that you put your screenshot in and try to scale it to be about the same size as the original image. if it doesn't fit try reshooting your image.
if you have is scaled properly select layer, and then transparancy on the top of the gimp window and click add. then pick color selector toor, select the background (if there is still too much background left turn the threshold up until you get the good balance)

now render the image and put it in C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\custom\custom hud characters\materials\console\characters\
if any of these folder are not there simply create them. if you want to replace a specific image change the name of the file to the one you want to replace.
if theres any info missing just let me know. i know there are no images but i will add them later.