1 you want to create custom monster and give him own graphic?
2 monster need to be placed on some map, i presume you dont want to make NEW map with your monster.
3 you need to replace existing monster. Which monster would you like to replace in which game? (doom, doom2, heretic, hexen, strife)
4 unfortunately i only play heretic, fortunately it doesnt matter its all the same.
let say you want to recolor gargoyle (this red flying imp) in first level.

5 download slade
https://slade.mancubus.net/index.php?page=downloads6. check out this zdoom wiki. Here is decorate code for Himp -
https://zdoom.org/wiki/Classes:HereticImpcheck "See:" this is part of monster behavior when he is chasing player.
IMPX - is sprites name
ABCB - is seqence of spirtes
10 - is the duration (time of display on screen) of every sprite
- Code: Select all • Expand view
States
{
Spawn:
IMPX ABCB 10 A_Look
Loop
See:
IMPX AABBCCBB 3 A_Chase
Loop
Melee:
IMPX DE 6 A_FaceTarget
IMPX F 6 A_CustomMeleeAttack(random[ImpMeAttack](5,12), "himp/attack", "himp/attack")
Goto See
7. open slade. open Heretic.WAD.

8. bottom left => Filter => type
impx*1

9. then type:
impx*2
10.
sprite name is like that f.ex
IMPXA2
IMPX - its the identification of the frame, like name John, Henry...
A - its the next frame. Usually you have A>B>C>D and this create animation.
2 - is direction of monster facing.

IMPX A 6
6 means time in decorate code, but direction 6 in sprite name11.
File> New>New Archive> ZIP archive
12.
save as > PK3 file
13.
create folder sprites, file a, and file Decorate

14.
in decorate write
#include "a"
save
15. Text language > Zdoom Decorate
set it for both files
16.
in fila "a"
- Code: Select all • Expand view
ACTOR Ciabata : HereticImp replaces HereticImp
{
Health 3
States
{
See:
BLUE AABBCCBB 3 A_Chase
Loop
}
}

you dont need to past all code. only the bits you want to change.
the first sentence means
Ciabata - new monster
: HereticImp - take all stats from gargoyle
replace HereticImp - on maps switch monsters with this actor (it could be also item like pick up f.ex)
17. in heretic.wad, select all impx A / B / C

18.
convert to GFX..
convert to PNG truecolor - at the top
convert all - at the bottom

19.
select again all the IMPX with * at the end
20.
r click -> export
21.
change graphic
upload into spites folder in
a.pk3

22. add a.pk3 to zdl

you can adjust the position of graphic in slade, so they not cut out in game (you will figure it yourself)
23. dont add half naked girls (unless you are 12 y old)