To my knowledge no one has ever put together a document detailing "how to" tie two dissimilar competing brand PBXs together. Even when you're fortunate enough to have two dissimilar machines that support (and interpret) the Q.SIG protocol identically, it's still a lot of trial and error. What works for one seldom work for another brand.
One end will need to be master and one end will need to be slave. The Mitel supports both ETSI and ISO and can do so either with E1 or T1. What you use will thus depend on what the other machine can do. You'll need the E1/T1 PRI module for the 3300 or else an NSU or else a DSU node along with either an MC269AA (E1) or MC270AA (T1) PRI card. With either the NSU or DSU option you will need to use IMAT software to configure the card. The internal E1/T1 module in the 3300 (optional) can be configured directly from the 3300 programming interface and so would be preferred if it were me. Note, each E1 or T1 will consume one digital link license on the Mitel.
The best "documentation" I have found is the Mitel PRI training course documentation combined with the release notes documentation for the specific version of PRI software you're dealing with. Current release I believe is 8.2.1.14. Still these documents are not going to tell you what the many esoteric settings need to be, but do a fairly good job explaining them.
Sorry I can't be more help.