it's easy. I would suggest filing a 1040x to amend your 2008 return. All you need is a 1040x, and 5405, HUD-1 form from home purchase (showing date acquired and price) and your last tax return. Basically you just fill out the 5405 (it's super simple), fill out the 1040x with the info from your last tax return... BUT on the back (or 2nd sheet) in column 'B' (the adjustment column) you add your $4k jointly, or $8k singly, then total up column A (original data) and column B (adjustments) and put that in column C. Do the simple math and you will come out with $4k, or $8k at the end of the form. In the explanation, just write "claiming 1st time homebuyer credit" or something like that. I don't have the forms in front of me, but that sounds right from memory. Let me know if you need any more help with it.
Oh yeah, make sure your 5405 date acquired matched your HUD-1 form date acquired. Easy!