i would think your first job would be to research the need for this and is there enough to make a living out of it.
if you find a lot of customers show interest you need to work out the logistics - where will the animals be?
do they come to you or you go to them?
you can probably only have one or two animals at your own house at a time - (assuming they are dogs)
also how many animals can you logically look after in one day? sick ones will need more time.
do you just pop in, feed them, walk them and leave or do you sit all day with them?
is it just during the day or over night?
daily or a few weeks at a time?
you may well need a police check certificate too - as people will be leaving you their house keys and will want some proof of your trustworthiness.
to find all this out i would suggest a leaflet drop in your area - and see how much response you get, and make a list of the sorts of animals and needs etc.
good luck