Drawing glass can be difficult, in this exercise two pencils and the lid have been added to show how objects distort though the jar. It is really important to consider value in all drawing but particular with glass, water and shiny objects. Value is the word for darks and lights. Getting glass to look right is all about analysing the value of the tones.
Another very tricky aspect of a glass or a jar is drawing ellipses...many of my students really struggle with them. These are the round openings of mugs, jugs, jars or any other round vessel, which appear oval if not seen from directly overhead. They vary in shape according to how high up towards your eye level they are, narrowing to almost flat as they rise and opening to almost round the lower they are. Comparing by eye the height of them versus the width is the key to getting them right. Never be worried about putting in light 'working out' lines, and although there is only one opening on the jar below, you can draw ellipses at any stage on the way down, for example at the neck and base to help you work out the shape, the lid is also an ellipse shape. Drop vertical lines down from the side to join up your ellipses and you have the rough shape laid out. If you draw a cross from side to side and top to bottom of an ellipse it should be completely equal looking in all four quarters. Also your base ellipse should look less narrow than the one at the top.
For the rest of the drawing you need to work out how dark and light the tones are. There are nearly always some very bright highlights which come from a window, that can be left as white paper. A good way to add tone is to then cover the whole drawing in the lightest possible shade of tone, then add incrementally more where it is slightly darker, and again until you get to the darkest areas. You can always rub away at the drawing to fine tune it. Your rubber is a tool not just a corrector.
Look carefully at where tone gradually lightens or darkens compared to where it has sharper edges, and if you can see an edge, don't draw it.
Examples from my classes...