There's a free program called House Flipper Workshop Exporter on Steam (you need to look for it in the store with programs showing, not just games), as well as tutorial videos for using it on Frozen District's official Youtube channel. If you search Youtube for 'House Flipper Workshop' the two videos should be in the top few results.

Uploading finished items to the Steam Workshop is the easiest part of the item creation process, however. To make the items, you need to make the 3D models and textures for them. There are a lot of 3D model making programs and each one has its own tutorials. These programs are often quite complicated and take a long time to learn. If you search Youtube for Blender tutorials, that will give you some idea as to what is involved. (Blender is the program I use to make my Workshop items. It's free, however it's very powerful and can take some time to learn.)

There are websites with 3D models that some Workshop creators download and use for their items, instead of making their own, and Unity has an Asset Store too. The Unity Assets tend to be lower poly, so work better in the game than the website models. Most of those are very high poly and not originally designed to be used for game items, so they can cause problems in the game if they're not converted to low poly models first. (Blender can be used to do that.)

I hope that's helped :)

Thank you Dark_haunt. Very helpful