How do I make a texture without a background?

[Godzialox666]

 like, I wanted to make some bars where there are monsters inside, but that the monsters can be seen and that they don't go through those bars

[Gradius]

Create the sector your monster will sit inside of, select the lines you want to have bars on them, then assign it a middle-texture (right click on the selected lines in Doombuilder) and choose one of the "mid-" labelled textures such as 'midbars1'. You probably want to assign both sides of the line the middle texture to avoid issues. Then select the 'Impassable' and 'Block Monsters' flags to prevent the player or monsters passing through the bars.