First - an iOS or Android smartphone with a circa 5" display. Anything above 5.5" is pushing it for the next item you'll need.
Second - Cardboard, of the Google variety.
Third - the OnePlus Two Launch app from the Google Play Store - https://play.google.com/store/apps/details?id=net.oneplus.two.vrlaunch
Fourth - a sharp knife to cut away all those bits of cardboard that dig into your skin whilst you're wearing it. I had an issue with the bridge of my nose, being of Viking lineage, being dug into by the three bits of cardboard in that area of the unit. A bit of swift sawing with a sharp knife dealt with that and now I could wear it for hours.
Fifth - Patience if you're in the Eastern to mid hemispheres. The event is timed to 7pm PST, which is 3AM British Summer Time, and no use to anyone East of New York.
Sixth - Don't look at any OnePlus news - more or less all the device specs have been revealed, which doesn't leave OnePlus much to do this evening/tomorrow morning except tell us what the software can do, and if it's running Oxygen OS you need only look to stock Android for inspiration.
Seventh - There could me more than one device, lets keep our hopes up.
Eighth - prepare not to be surprised if it costs more than the last one - yes, it's still using the same screen as the OnePlus One, it's still the cast off's from the iPhone 6 Plus (they're the same screen, hence the delay in production of the One last year when Apple were ramping up production of the 6+, at least that's what I believe to be the case), but it's also using more RAM, a newer Snapdragon chip, and a bigger battery. These costs add up, and if OnePlus have any sense they will also have paid for better quality control, better customer service, more service centres (we've seen the plan for seven in India alone), and better support, which all need to be paid for by the device's initial cost. Plus they are also making their own, albeit barely customised, version of Android, and programmers cost money too.