Quite an easy step!
Just upload your source rpm (mypackage.src.rpm) to ftp://ftp.linux-mandrake.com/incoming/ Then send an e-mail to lenny in order to warn him. Additionally, you may want to CC it to the "Cooker" list.
If it is just a small change to an existing spec file, or if you have just added a patch, usually sending in the spec difference file and any additional patch file will do.