https://docs.google.com/spreadsheets/d/1YlItpJB5cKRYUAioyy6TSZDHwaAiAWOZu4oamAySqJA/edit?usp=sharing

The above link should work. It's with a throwaway account. You'll need to make a copy into your account to edit it and play.

You should make all your changes to the green boxes. I think they are pretty self-explanatory.

Under "Payment" there are two numbers. The top one is calculated (don't change it), the bottom one is what you can change if you want to pay a different amount each month. Say you want to round up to $1,000 instead of $944.40 in the case of the original poster.

To the far right of each month (under "actual paid), you can put a value there. If you have something there, the top payment amount is ignored and that amount is used as a payment instead. Leaving those blank (which is the default) will mean you pay the amount in F3. If you leave them blank and leave F3 alone, you will get the scheduled payment according to the bank.

You probably shouldn't change anything that's not in a green box, unless you know what your intent is.