I wrote mine a long time ago and given the variety of forms and expectations those applying to complete their PhD are dealing with now, I would hate to get too specific with you. Do you have a Graduate centre at your current university? I would speak to them and see if they have any exemplars you can look at and if you know anyone at the specific universities you are applying to, talk to them about their proposals and the specific proposal process at that establishment. At my university, you can get some assistance with writing these documents and I, and my colleagues, are usually very open to helping students to develop their proposal. Even if it isn’t with our university.
Do you have a particular professor you want to work with? If you have already been in contact with them and they are interested in working with you, ask them for guidance too. They should have a good understanding of the expectations and process at their university. They have a vested interest in helping you with this process if they want to supervise you.
When I have to write this type of thing, I often use Google to see if I can find any examples I can draw on. The key in anything of this type though is to be specific. I just found this advice from a UK university for prospective postgrads writing a personal statement Have a look through a few of these types of things and then look at the specific instructions from the university you are applying to.
Good luck. My background is communications and it is a fascinating and fast-moving field.