First of all you will need Hotline Miami's Level Editor (Because trying to do it without would be a waste of everyones time, brain cells and chromasomes!) and Notepad + + !

If you do not know how to edit items in Hotline Miami's Level Editor with Notepad + +, I highly encourage you to check out Yossarian The Assarian's 100,000,000,000,000,000,000,000,000,000 guides on how to do exactly that, here is his most famous which is basically "The Miami Encyclopedia" ™ ©

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=676175384

And could I really forget about Rob and Matt's Guide to Hotline Miami Modding? The Dynamic Duo may help you with their video series that will help you mod!

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=966323230


Also for reference on what numbers are what, here you go!
The Sprite and Object IDs! Even though you could just get them at "The Miami Encyclopedia" ™ ©
https://www.dropbox.com/s/pb1jtorihy4o4b2/sprites.html?dl=0
https://www.dropbox.com/s/1br7tkh4nrt9u0e/objects.html?dl=0

So! What you are going to want to do is place in a dummy item that you want to change into one of the walls such as a Vase Stand (IDs - 229, 121) and save the level. Next you will want to go into the level's .obj file (It is different depending on what floor you put the item in, level 0 being the lowest. Once you are there you will want to find the IDs for the wall you want to use and put the IDs in for their respective Sprite and Object IDs instead of the original dummy Item's IDs.

Once all that is done, save the Notepad and go back into your level (You will want to exit the level and go back in first to have the effects kick in, any editing to the level before you do this will interrupt this process and put you back to step 1)

And there we go! You should find your wall that you can drag around as if it is an Object!

So, gonna get the bad things out of the way first! Sadly there are No Corners when you do this method for the walls so if you love corners then I am sorry for your loss, other than that however nothing is different!

As for the technical stuff, There are multiple different "State IDs" as I label them for walls that do not go under the normal name for the Wall's Sprite IDs, they don't explain what they are or anything like that but they are there, from what i know they are used to determine the size of a wall or conjunction such as four wall pieces vertically and the different types, this is only for the fixed walls however in .wll

UPDATE: Found em' In Sprites!, They are designated with numbers beside them such as Brick WallH128 which is a 3 wall piece going horizontal (As it has H), You can find what they look like in The HLM WAD Explorer under the Walls/BigWalls section, Here is an ID of the Brick Wall H128 to gets you's started! (2967)

Thanks for putting up with the seconds it took to read the guide! Sorry for the lack of pictures aswell!

Credit to Yo Boi Skeletor for the comment on my placing special doors guide that just inspired me to do this for no reason whatsoever