Basically the drawing board is your tool, you work from initial aerodynamic shapes, and construct the aeroplane on paper, on a series of sheets of paper - the bigger the better. Working in 3-view, you put faint working lines across that allow you to join everything up. It is reliant upon having a good and continuously developing mental picture of the aeroplane or component you're designing.
I worked for a while in the drawing office(s) at RAE where I did a fair bit of this in the latter days of pencil designing when CAD was starting to become new - and happily designed quite a lot of complex shapes.
Genghis Sr. was for rather longer in the Vickers Armstrong drawing office at South Marston where he worked on wing sections for aeroplanes like the Attacker and Scimitar. Comparing notes with him from time to time, his tasks were similar - but those chaps were very good engineers, with an excellent mental picture of what they were designing. Also however they had A LOT of people - think large rooms full of drawing boards, full of intense young men with thick framed glasses, slide rules and collections of data books and drawing instruments on the side table. The amount of training they had, and the level of skill in the task, was arguably rather greater than many CAD operators nowadays.
Mike Whittaker, who's still out there designing using CAD these days, designed his earlier designs just this way - (
Plans | MW Club if you'd like a set to look at) and he'll send you a complete set of drawings for any of his designs. I used to use them for teaching aircraft design when that was my job and very good they were too.