Try to break it down into years and then months within each year, or at least blocks of months. If you are applying for funding, or if the law department where you are applying has a set time for PhDs work with this timescale. Year 1 for example, background reading, literature review formulation, application for ethical approval etc etc. Year two, commence fieldwork or source data etc. Year three commence write up. If you have classes that you have to complete, include these also. Nobody will 'hold' you to this initial timescale but it is a good exercise in focussing the mind.
I did this in year 1 and submitted the last day of year three - as per my timescale;-).
If you are familiar with Gantt charts you could use one of these, or maybe Excel. It doesn't have to be on a spreadsheet but I think it helps convince the authorities that you are organised and have a good handle on how the PhD process unfolds.