Same way you'd do it with a frame that had the same size tubes. I have a SS/FG where the head tube is bigger than the top, down & seat tubes, which are all the same size. Anyway, the middle of the head tube should align with the middle of the seat tube, provided the seat tube is welded in the middle of the bottom bracket. Without the wheels on a line should be able to be drawn from the bottom of the middle of the head tube to the rear of the bike. The dropouts of the fork & rear stays are 90* from that line and equidistant from the line on each side. With the wheels on the bike if all of that is aligned and the wheels are laterally & radially true the bike tracks a straight line.

|------|----------|---|

rear dropouts => seat tube/bottom bracket => Head tube => fork dropouts

trued wheels on that line and the bike steers & rolls in a straight line.

Edit: This is also the way you could do it for a BMX frame.