How to make custom sprites appear ?

I assume you're using slade, right?

I'm not sure what the mistake you're having is, but you should try to make a new actor instead of trying to replace an alrewady existing one. Try this:

1.

Name the picture you want:  "MYDGA0", or something along those lines. What matters is that it is 6 characters long, and that it ends with "A0".

2.

Write in the decorate file:

3.

Make sure the sprite is between an "SS_START"marker and an "SS_END" marker. You can create such markers by clicking the "sheet of paper with an E" Icon, right below "tools", and naming your entry "SS_START" and "SS_END" respectively.

4.

Left click your sprite in slade, and below the display youll find the "Offstets:" configurations. Select "Auto" and then "Sprite".

Once you've done this, if something still doesn't work, try going to realm 667 and downloading some of their sprites. Then open their sprite with slade, and try to see whether you did everything right. Also, the red square with a yellow exclamation mark usually means there will be an error report in doom builder, in the lower right side of the screen. Telling us the specific errors you're having would help us understand your problem better.

It did indeed work. Sprite was well-named, DECORATE was working. I just, hmmm, forgot the markers. I guess this because I'm used to ZDoom mapping and therefore don't need PP_START and stuff. Thanks a lot !