Yes, I tie them first, unless it's some knot I know\remebered rather well. Some real knots have irregular structure depending on agle you see them from, so I do that in real life first time, using a thick (8-10mm) rope, preferably colored.

Anyway, here is manual for simple, so called reef, or square knot 9boy scouts should know that one?) Although it's often used to twine ropes together, or to continue doubled rope, when it ends too early (in that case one rope would get small knot and tied off ends after.

Now about drawing itself - there in lower right corner of picture:
1) I markup where ropes should be, where they head to, where is intersection is, that will be the knot.
2) I draw _solid_ thick lines, by hand or using splines, depends on scale.If setup is complicated, it may happen in different layers. On this drawing I used different colors for two ropes - in some editor programs you can assign overlay color to layer and draw just in back, switch it off after
3) I do some adjustment to rope lines - ropes may overlap, squeeze into material underthem, disappear beyond corner\curve of object they wrap around - that affects thickness of line
3) I use eraser tool - it's handy that I have it on other end of stylus - to trace along those thick lines. This way I can control thickness of "rope" - gap between lines. Brush is set to minimal line thickness I use - this way, rotating stylus around I can switch from erasing to fixing rope's outline in less than 0.5 second. Heh, some people ask why I whine when some editor doesn't support eraser on stylus or separate brushs settings.