POST syntax for form-filling

I'm writing an itsy-bytsy teeny weeny HTTP client which (with help of a
nearby Squid proxy) is able to fetch any URL without even having to use
a resolver (it's for a small device). The only thing missing is how I
build the POST request to simulate form-filling (GET is trivial). The
HTTP 1.0 RFC wouldn't help. Where's the authoritative info?

Thanx all,