Honestly, it's a lot different than what's in the finance books. Banks have an ingenious way of twisting the simplest things.
So I had to help my friend replicate the repayment sheet and calculate the real interest rate she's paying. In the end I saw an awesome template online that put mine to shame.
I modified it slightly because it was perfect already and very beautiful. It will adjust dynamically to the payment period specified. Just open it to see what I'm saying.
The template is from http://www.excely.com/ and the source download link is Loan Calculator
But my modified version (mostly making it show in Naira; you'll be amazed by how well it does that) can be downloaded at Nigerian Loan Repayment Calculator
The only issue is you have to key in the management fee and VAT payment yourself. The way the banks calculated them is not consistent. So I didn't want to lock in any formula for it.
And I hope you won't need to use this. The loan rate I saw for one the banks is 23.99% and with this repayment plan (monthly withdrawal) it's not even good for business use. Any business that can generate enough to service that kind of loan is worth patiently building/growing without the loan.
But if for a valid reason you are considering taking a loan of this type, especially mortgage loans, then this will help you do the maths in less confusing way and show you the sum of all the extra payments (interest, management fee and VAT) you are paying the bank.