I have experience making textures for items that already have a UV map, but this is my first try making one myself. I apologize for not being able to more precisely describe what I want to do; I suspect that not knowing the terms is hampering my google searches in an attempt to find the answer on my own.
I have this model, made in SketchUp, shown with textures: https://gyazo.com/f6585ce8948666f694c8abd05701e609
When I unwrapped it or otherwise had Blender make the UV map, it created a lot of non-overlapping rectangles, one for each face. However, I liberally re-used textures when making the lantern, so that was not at all an efficient use of the available space. So, I made a texture map by hand that is much more optimized The problem is, I am not seeing a way to tell Blender which areas to use on which faces of the model, and all the tutorials and the like I am finding are trying to help people with more conventional starter tasks, like upholstering a chair or making a shirt, where what I want to do would be insanely fiddly.
Is there a way that I am missing to tell Blender the intersections for the UV map, or am I doomed to having it tell me? I am aware that in many situations this is precisely backwards, but for this project, it would be a great boon to my final file weight and texture quality. I am willing to use programs other than Blender if it turns out this is a thing you need Program X for.