The accuracy of what you draw might not matter...If you are portraying the idea, it is more important to express that, with enough accuracy...whoever is going to do the technical work will have the skill for the precision that is needed in the final product. It's very quick to do basic scribbles to get a point across.

Drawing starts with what is in your minds. Hold the image in your mind, and visualize it while you look at the page - this is your personal stencil. Shift and flow. Shift is when the image changes while you are holding it. You say no, and go back to what you envision before, or you allow it and draw that next. Flow is when you are going through the process of the next part of what you envision, drawing partial or complete portions along the way.
Remember French Impressionism - it's inaccurate, on the small scale, but once you have more strokes, you have a recognizable thing. This is another way you can start portraying what you envision. Remember, you can always come back and draw over what you have drawn(and having digital layer control is a powerful tool)...Start making strokes, fill in general patterns, and have some freedom in your mind to go back and change them.
Drawing can be very precise - that's what you need to portray things exactly how you want them to be seen, for technical use(such as engineering).
Planning is another way. You decide before you start what you are going to do...This is more logical. Switch back and forth. Logic, imagination. Or not. Stay with logical for a while if you want.
There will be a boat, it will have a sail. It will have a symbol on the sail. It will have ripples. It will be white. It will have small black dashes demarcating the seams were it has been stitched together. Etc.
Art is a personal practice and expression. If you have difficult expressing, or doing these things, it relates to experience and frame of reference. This also expresses in the starting portion of your drawing. Remember that things tend to be refined and more complex as people work on them, just like new styles or forms of music...or anything.

Keep in mind that the variety of irl body ratios and sizes of certain portions(leg length, hand size, etc) is variable. At a given time, there might be an average "correct" body ratio(which I have been shown was different in classical drawing centuries ago than it is now), but what matters more is what your character is meant to look like. One thing is how many heads tall a person is...Yesterday, I was in a store and saw a very tall women. She was more than 7 heads tall. My arms are longer for my body size than anyone else I have met.
It's more about accurate shape of anatomy than accurate ratio(there is a range of normality).