Along with getting yourself prepped and sorted, you need to be thinking about your business and keeping that organised.
When working on a job I normally buy Volant Moleskine notebooks as well as plastic A4 & A6 button wallets
Any paperwork I acquire over the course of each job I keep filed in one A4 plastic wallet.
Label each wallet with the job name.
Each receipt will have the name of the job written on the back (just in case a stray receipt from another job goes into the wrong job wallet).
Any receipts I have for that job will go into the A6 plastic wallet then into the A4 wallet.
Why a Volant notebook?
I like to keep each job separate and I like to file away the whole job once it is complete, rather than having notebooks, paperwork and receipts all over the place.
In the front of the notebook I would add:
Job Title -
Job Number -
Booked By -
Dates of the Job/ Date Booked -
Number of Looks -
You could of course write all your notes on a phone. I prefer to have this kind of information in analogue, and something I can go back to if need be, months or years later.