https://www.mapeditor.org/ — "Tiled", to create the map itself
Any graphics editor software to edit the textures

You can find a lot of videos about how to use Tiled on YouTube, or you can learn how to use it by yourself all alone. Examinining source files (.json) of maps other people made could be very helpful:
https://cdn.discordapp.com/attachments/574968748281167880/575389093719048193/maps.rar
https://cdn.discordapp.com/attachments/574968748281167880/575448922739310629/ses_ek.rar
https://cdn.discordapp.com/attachments/522796725279391744/574957561133137960/pkp_oldclassic.zip

Outside information you could need is the objects list used by the game and other information.

You can find objects manual here:
https://cdn.discordapp.com/attachments/574968748281167880/576495851564826634/DLIRIUM_Object_list.txt
Object manual was written by Xeren.

All area is being set to void by default. If you make a map, you could be walk on it, but if you try to use a teleporter, Ada will blow up into pieces. To prevent that from happening, you need to mark player allowed-to-teleport zones using same rectangle as same as you mark the walls, but instead of using obj_wall, you need to set such areas to obj_checkbox.

If you get a "ds_list_find_value argument 1 incorrect type (undefined) expecting a Number (YYGI32)
at gml_Room_custommap_Create" fatal error, then your map size is set to Infinite. When creating a map, you need to set a custom size for it, but NOT Infinite. If you already created your map and just noticed it is infinite in size, all you need to do is (in Tiled) click "Map", then "Map Properties" and uncheck "Infinite" on your left. If you would like to change the size of your map, you would need to edit "height" and "width" parameters in your .json file using any text editor tool (such as Notepad).

The first time you load a custom map, game converts it to .dmap format which is located in C:\Users\ -your Windows username- \AppData\Local\Delirium\maps. If you play your map, then make changes to your source .json map and then play same level again, its old version would load up, since it would be loading from previous said .dmap file. To make it load the updated version, delete that .dmap file.
Update: you can now delete .dmap files in-game. In map screen selection, choose the map you want .dmap be deleted for and then press Del button on your keyboard

Do not flip tiles as it would result in game crashing (tested 27/06/19, may be fixed now, but still be careful)

To upload your maps to workshop, you need to do the following:
1. Create a new folder in your game directory and name it "UGC"
2. Move (or copy) your map and it's preview image (the one that will appear as its preview in Workshop) there.
3. Rename preview image to "promo.jpg". The image must not be larger than 1mb in file size.
4. Launch the game, proceed to acrade mode and then click "Steam Workshop" in the top right corner.
From there, you will be able to set up your workshop submission and post it.

You may need to accept Steam's "Subscriber Agreement" before your uploaded map could be actually published

D'LIRIUM has it's own official Discord server. You may share your maps, ask for tips and all other such things there!
https://discord.gg/UzZsuYK