I would start with policy and procedures, then move on to documents. Most of the items that would show up on a checklist should be covered in those areas.
So go through loan policies and procedures and see what is suppose to be done. For us, a credit report is to be pulled and must be within 60 days of funding date, add that to the checklist. We have operating procedures that cover how long funds need to be on deposit before doing a savings secured loan, so that would go on the checklist. Then move on to what are the required documents for each loan type. All loans would have an signed application, funding docs, etc.
We have a customized checklist for vehicle loans, personal loans, savings/cd secured loans, credit cards and indirect vehicle loans.
After you start reviewing a few, I think you will get an idea of what you are looking for. For us, it is an always moving target, so whatever you create, just be sure to be flexible and willing to make changes to it.
Hope this helps a little!