I always try to start with the largest proportions first, usually the contours. Proportions are simplified into large planes, represented by thick juicy lines. These lines more or less give you a margin of error and are easy to erase. Think of lines like relationships. Soft thick lines are non committal, like a floozie you would meet in the back alley of a truck stop. When you make hard dark lines on the other hand, you are pretty much married to as it is hard to get rid of. The lines shown here are made by holding the pencil from the back, which gives full range of motion so you can draw from the wrist. As the block-in advances, the lines become more refined and committed.The beginning of a construct is where I do most of the measuring, usually with a plumb line or the back of my pencil. Height and width are always the first two things I measure. After awhile, measuring is not as useful as merely making visual comparisons. I remember back in my teaching days I would tell someone their proportions were off in their completed construct and they would respond with "Waaaaaaaah, but I measured, waaaaaaah." I would always have to explain that there is quite a bit of room for error when measuring with tools, and therefore your eyes are more accurate. Afterwards, they would worship me for my insight and flog themselves with sticks for ever doubting me (at least, that's how I am going to remember it in my head.) In addition, visual comparisons are also extremely easy when sight sizing. Using things like a black mirror are a huge help as well. Also, abstraction of shapes can also be helpful (instead of thinking of a shape as an ankle, think of it as a dead baby penguin.) Disassociating yourself from what something actually is enables one to draw it more objectively.
After the contour seems accurate enough, and I stress enough as its not going to perfect, I begin to work more interior landmarks, and eventually the shadow pattern. Always simple to more complicated. Larger lines to smaller ones. Below is a good example of a block in by Charles Bargue from his drawing course. If you haven't picked this book up yet, do it.
Clearly this drawing isn't meant to win any beauty competitions, but it will be very useful after I transfer it and start painting.