I've talked about this drawing exercise before, but I am so convinced it is great practice I will share it again. It comes from the great William Whitaker so it can be trusted. Note, this is not how I learned to draw but it would have saved me a lot of time on the journey. I've included some examples below. They are rougher and I cheat a little on the measuring after doing the method a lot but it gives you the idea.
step #1 - Get a pad of tracing paper. Set it up on an easel or whatever, close to a photo/model that you want to draw.
step #2 - Draw it. Try to do as much measuring as you can. Measure the distance between features with a straight object. Stand at least arms length, close an eye, hold your arm straight and use the end of the pencil and your finger (this will be a usable technique for life drawing too). You'll get something like the example #1 below.
step #3 - step back and squint you should be able to see some structural problems. Tear off the drawing and put it underneath the next page. Trace only the correct lines and pull away the drawing #1.
step #4 - With those lines as a base re-draw it. You should get closer this time around. Continue to step back and squint to find all the problems.
step #5 - Repeat 4-5 times.
Now this is never intended to be a nice finished drawing and it's not beautiful, but it should help get the right proportions, you can transfer it to good paper and do a nice drawing or like me use it paint for a painting.
You are learning to see. Most people can handle a pencil fine but seeing where a line should be is more difficult and takes a lot of practice. Another very important trick for drawing is putting a line down. It doesn't have to be right just put something there and you can then use that line as a fixed point to decide it you are too low/high/long/short/left/right.
Also, this is the basic measuring process I use for every drawing but all together at once on one page and without the tracing paper. This is just a good way to break it down and take a more analytical approach which helps with learning and makes problems more apparent.