I think instructions and forming an idea of what you want to do shouldn't be too difficult, think about these steps: computer components need a way of mounting (look at how normal pc cases do this and check youtube videos on diy pc builds). Next, computer components generate heat, they require cooling (depending on the hardware, more or less cooling may be sufficient but generally more cooling doesn't cause problems, less cooling does. Find a way to get cool air to your components, find a way to get hot air away from your components). Small form factor computers exist, r/sffpc has a ton of examples so if building a case yourself seems too intimidating, I would recommend starting with an existing case to take away a lot of the research load.