Before paying a debt collector that sends you a letter or calls, you should first make sure that you actually owe the debt and check to see if the debt has passed the statute of limitation which is 7 years.
If it's been 7 years or longer since the debt was owed and the debt collector is still trying to hound you for the debt then don't pay.
Because after 7 years a debt collector cannot force you to pay the debt or even sue you for the debt.
If you do still owe the debt then keep calm and don't cave into any pressure from the debt collector, get things in writing and either pay off the whole amount of if possible setup a payment plan with the debt collector and make the on time payments.
If you fail to make the payments then the statute of limitations reset and you can be chased again for the debt for another 7 years.
Also before paying the debt collector make sure that they will remove the negative remarks from your credit report and report the debt is paid in full.
Some debt collectors will do this while others won't bother even reporting the paid off debt to to help your credit score.
If they won't report the paid off debt or remove the negative remarks then don't pay them anything because paying the debt collector may not even help your credit score.
When you owed the debt to the original company your debt owed is usually sold off to a debt collector who pays less than the debt you owed and then they benefit by profiting from the money you've paid to them.
As to how to actually pay the debt collector they will usually send you a bill in the mail or you can make payments through your debit or credit card either online or through the phone.
Also before making payment research the debt collector company to make sure they're legit and are not scamming you.