Trouble is, to make a model you do need tools- or they help.
Some kind of simple indexing arrangement, perhaps a fairly simple jig for grinding lathe tools, a depth stop is very handy, a handy lock lever for the saddle, perhaps a samll boring and facing head, or at hte least a small flycutter holder. Oh an accurate tee bar for squaring up up vertical slides quickly without having to change to a faceplate. A little holder for small milling cutters, and a drawbar for a small milling chuck.
Keep anyone a bit busy. My list included a GH Thomas Versatile dividing head, which, apart form the dividing/indexing, is one of the mose useful toolholders for slotting (before you make a little slotter) and of course for milling because anything round up to 3/4″ is automatically at centre height. Thats a great timesaver….Where do you stop?
If one is honest, even the most complex bit is only a combination of several single operations. The suggestion would be to think each operation through, whats to be used as a datum, and above all, how it is to be held for each operation.
But there is always a problem if you cannot viualise the object in your mind….so it might be worth looking at a small toolmaking project which will ease life in the future, but which comes with an isometric drawing -thats like a 3d drawing/pic rather than the old style single plane first or 3rd angle projections. An isometric drawing is a lot more user friendly and you know what the finished lump should look like. You should avoid end making a left hand version of a right hand bit!
Hemingway supply that sort of thing, and a good set of instructions (usual disclaimer) and I imagine there are others too … oh and get Tubal Cain’s book, Workholding in the Lathe. (absolute Godsend, along with everything else he wrote!)