IMO Magnussen's is the best, it actually looks like a proper circuit.
https://www.desmos.com/calculator/j9kv6hrwnl
Took some inspiration from Spa (Pouhon at the bottom of the circuit, Eau rough/Raidillon turns 2/3/4), COTA sector 1 (turns after 'Pouhon'), and also Monza, with a mini Parabolica.
The width is 12 meters throughout, and the length is about 5.5km.
Possible thoughts for elevation change: Uphill through turns 2/3/4/5, downhill through the 'teardrop' corner (turns 6/7/8).
https://www.desmos.com/calculator/0hvcmtjynw
I intended higher ground to be on the left, but I guess you can decide for yourself which way should be uphill for the flow of the circuit.
Featuring a reverse Spoon curve rip off (turn 14/15), a 1km straight and a few heavy braking zones.
Length is approximately 5.4km, the track width is a constant 12m.
You want to use a vector program else this is not going to work. Although the old string method works on everything from paper 1.0 upwards.
Anyhow, all vector programs have a function that gives you the length of any path in your drawing. For measuring a track, you'll need:
Let the program measure both the ruler and the centerline. That gives you the values c_px and r_px. And now simply plug it into the following equation to convert it to whatever unit you know your ruler in
c_m=c_px/r_px*r_m
Where m subscripts are reallife units and px stands for pixel ie what you get from the graphics program.
Obviously this works the same for alternative layouts, here your ruler is simply the prime layout that you decreed as having a certain length.
If you need a vector program Inkscape is free and does the job well enough.
Here is the original Desmos graph.
Turns 4/5/6/7/8 adapted from the Red Bull Ring, and the turns 12/13/14 are similar to Shanghai T11/12/13.
Track length: ~5.6km
Track width: 12 metres
Straight between T8 and T9: 700 metres
Flag run-off areas: Tilke
A-level preparedness status: Under
https://www.desmos.com/calculator/grjy898vdo
Influenced by Silverstone (spot Maggots/Beckets/Chapel) and Circuit de Barcelona-Catalunya (turns 14/15/16).
Main straight length is 1000m for reference.
Aiming to add more, we'll see if I can pull it off in time
A lot of bezier curves in Inkscape - there are tutorials in the wiki! I create separate layers for the main track, alternate layouts, runoff, gravel, etc, but better illustrators like /u/Cyclone1001, /u/girlwithaguitar, and /u/MMuster07 can probably give better advice.
I just use paint.net because I don't have Photoshop yet, though I've used Photoshop before and its a lot easier. I would recommend Photoshop, but if you don't feel like paying for it, use paint.net