Manually calculating weights for WLS

#1
*Posted in Stat Consulting but moved here b/c I think it's more fitting.

Hi guys,

I'm trying to figure out how to manually calculate the weights used in weighted least squares. I'm regressing three independent variables on y using actuals, but have no other information than that. I need to be able to do it by hand (or via Excel), not using a stat package like SPSS. Thanks in advance for any help you can provide! Let me know if any more information is needed.

I've come across a number of "methods" for determining weights by scouring the internet, but none have worked properly. Not sure if I need to do matrix algebra, or multiple stages of OLS regression, or some other method...

-Brett