OP, if you haven't done so then start by downloading the latest RSPO certification standard from the RSPO site here and having a read through: https://rspo.org/resources/
There are similarities with food safety certification schemes, in that it expects you to have written procedures, records, training etc. If you are lucky you'll be able to find whatever your predecessor did on these and build from there, but even if not, the very narrow scope means it shouldn't take too long to create something from the ground up if you're used to writing procedures for other certification standards. I expect you'll need e.g.:
A procedure to ensure you purchase from an RSPO certified source, and for checking that this is the case.
Training to the purchasing (and possibly warehouse) teams to ensure they know what they're supposed to be buying/receiving, and obviously a written record of that training.
For the management review part I'd just add that as a brief section into your existing management review process.
Similarly for the internal audit element, I'd think this will be easy to cover as it's such a small area, so just get it added into your existing schedule.
I suspect the "calculations" are the reconciliation discussed in C.3.1 and C.3.2 in the standard. In your case these should be very easy as you're only using one ingredient with palm, so 100% of your materials are RSPO mass-balance certified (assuming you're buying from a certified supplier ). You'll just need to ensure this matches up with your company's accounting/invoicing as covered in section C.4, so I'd make sure to get people involved from that team - they're the ones who will be responsible for maintaining/providing a lot of the relevant records.
As you can imagine, these calculations become vastly more complex if you're buying multiple ingredients with palm derivatives, plus palm oils, some of it is RSPO mass-balance, some of it is RSPO segregated, some of it isn't certified at all etc...
For your situation IMO it really should be as simple showing that you bought e.g. 100kg of MB certified sprinkles in a 12 months period, and sold product which used 100kg in the same period.
First question...
If this is 1 ingredient and your supplier is already RSPO certified why is your company seeking RSPO certification?
RSPO is a bit like organic certification (without the formal regulatory requirements), in that each stage in the supply chain needs to be certified in order to maintain the "claim" around RSPO status and pass it on to the next stage. Lots of consumer-facing brands and retailers (at least here in the UK and Europe) want RSPO certification, which means every stage in the supply chain prior to them has to be certified.