A company wants to pay me for project to clear their books for 2020. Project will not start until 2021 how do I proceed?

[Opus 17]

Unless you are an accrual-based taxpayer (this is complicated and requires an accountant so you would know if you are), then you are considered a cash-based taxpayer.  You report income when you actually receive it and you report expenses when you actually pay them. 

[Anonymous]

what exactly do you mean by clear their books?    will you be an employee or an independent contractor?

if an IC consult a tax pro if the amount is significant because you may want to elect to be an S-Corp for which you should get pro advice.