Releasing one version of a Scientific Paper under an Open License
A Creative Commons License is a good way to achieve this.  It doesn't really matter how you convey this information, as long as it's clear.  (A footnote on the first page is fine, for example.  Whatever you prefer.)  The attribution license (CC BY) sounds like a good choice for what you're looking for.

However, this definitely conflicts with the copyright form.  You can try asking the IEEE for permission, and there's some chance they will agree, but they tend to be picky about copyright.  If you can't come to an agreement with them, then you will have to decide whether publishing in this conference is worth giving up copyright, and they will have to decide whether it is worth losing your paper over this issue.  (If you have already signed the form, then there's nothing you can do except ask them.)