development of payment modules

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I dont want to crosspost, but i tried searching the forum and came up with nothing :(

Is there any documentation available about how to write your own payment module?

I really need to create 2 payment modules, and i cant find any similair modules in the contributions.

What the first module has to do is the following:
-Let a customer choose if he/she wants to pay in 12,24,36,48,60 or 72 months and [i]automaticly[/i] show (while the select) how much they pay per month (lease system)

What the second one has to do is rather simple i guess:
-Just add 25 + 1,5% of the subtotal.

Anyone who can point me in the right direction about how to manage this/where to find docs?

Thanks in advance, zlack