I start out a drawing a straight line longer then the fuselage along the center of the paper. Will call this the thrust line and it will go from the engine mounting to the tail. All of the bulkheads, and the firewall will be at right angles to this line so when you draw them, you will know the measurement above and below this line. If you have a set of plans from one of your kits, you can get an idea of the spacing of the bulkheads, wing ribs etc.
You can draw the fuselage on the plan and get all your vertical measurements from this. When you draw a front view, you will get all the horizontal measurements.
The wing is the same. Draw a single line across the paper and make this the " spar " line. Draw out the ribs, the tips, the center joint. You will be able to get all your measurements from this to make the spars, ribs, tips, braces, etc.
After you decide on the type of airfoil to use, and if you are making a constant chord wing, you can make a Master Rib out of plywood. Using the length of rib from your plan, draw out the rib on ply and cut it out. Now taking balsa of the right thickness, cut blanks a little bigger then your rib. Stack these blanks with the master rib on top, drill two 1/4 inch holes through the master rib and through the stack of blanks. insert two 1/4 inch dowels through the holes to secure the stack. Using a band saw, you can cut all the ribs at once. then you can sand and cut the stringer slots, and all the ribs will be the same. This is a lot easer to do, then it is to explain !
You draw out the parts on balsa using the measurements from your plan, cut out the parts and build the plane as if it were a KIT.
For me computer drawing is a lot easier than freehand because you can move, bend, cut, paste, slide, join, trim, scale, etc with a single command.
Starting the same as any drawing, decide on your dream plane, then start laying out the lines on the screen. If you have a CAD program, you know how to do this, if not this is just a general idea of how they work.
First I make a starter drawing and set the screen up full scale, like 3 1/2 X 5 feet in size, then zoom in so that the full scale will be viewed on the screen. I save this blank drawing to file as a starter. Then using the starter drawing, I start the same way by laying out the thrust line longer then the fuselage, then continue laying out lines, curves, circles, etc. until the fuselage is done. Then I add the dimensions to all the elements of the drawing. You then save the drawing to a file name of your choice for later use.
The next thing to do is call up your starter drawing and draw the wing the way you want it, lay out your dimensions, and save it to file.
You do the same with all elements of the plane until the ship of your dreams lay before you! I know this is over simplified, but if you get a CAD program for your computer, the basics are pretty easy to learn.
After your CAD drawings are complete, the next step is laying out on a full size plan.
Starting the same as freehand, with the poster paper laid out on the table full size, draw out the fuselage thrust line the full length of the plan. Then draw in the firewall using the dimensions from your cad drawing, then the bulkheads, and all the elements using your dimensions. In a few minutes you will have a full scale drawing of your fuselage.
You draw out the wing, tail section, etc. the same way. In no time you will have a plan that you can build your plane from.