Start by saying thank you if you know that the person has already agreed to help you. End with a thank you as well.
Use bullet points or a list to present the instructions clearly for the reader.
Phrase the instructions as polite requests, e.g. Don't forget to ... , Please can you ... , Would you mind ... However, you can be more direct for important instructions, e.g. The ... need(s) ... , Under no circumstances ... , Don't forget to ... .
To emphasise the importance of an instruction, explain what the consequence might be if it isn't followed, e.g. under no circumstances should the rabbits be allowed out of their cage as the neighbour's cats have attacked them in the past.
Let the person know how to contact you with further questions if they need to.