You can contemplate it starting with 10-15 minutes or so and see how it goes, you might need inspirational material so you have a foundational structure to begin with, to keep you busy, preventing you from running out of thoughts.
ie one could start out thinking along the lines
"Death will certainly come to me!"...Life is uncertain,Death is certain!... All that is Born must Die"
So one can spend some 3-5 mins on this before moving on to next theme.
Thinking about how other people could not escape Death and its inevitability, ie The Buddha, The Great Arahants, one's own dead friends & relatives, music or movie stars, or other human or non-human beings for 3-5 mins before moving on to final stage.
Contemplating one's own death, along these lines "In many ways can death come to me..." and the drawbacks of going on to be reborn.
All of this should be done with Death potentially being close in mind, not 10 years or ten months from now, it can come before one finishes the meditation or this very day one can fall and die etc.
There is no need to divide it like this in stages but it will be helpful to develope the meditation and one can adjust it later as one wants. One can pick and choose which way one does it and overlap is natural between "the stages" and it will get repetitive if one finishes and starts from beginning for a "second lap".
So one should be picking the most appealing themes and dropping the stuff that doesnt do much for you after a while, elaborating, poundering and visualising, developing the meditation to last 30, 40, 60 minutes or longer.
I think one should make an accent on thoughts of compassion in regards to one's own body when contemplating death, the body is vulnerable and is very fragile as it is so one should take care of it. One should contemplate in spirit of compassion and appreciation of the body as a vehicle to Awakening.
This is a part i would not dare to neglect.
Thats all you need to do, keep contemplating it over and over, it will eventually lead to incredible fruits.
Look up both Maranasati Suttas 1st and 2nd, as i understand according to Sutta one is adviced to do it in the morning and in the evening.