1) Sit in orbit 5 minutes before Iron Banner releases
2) Once IB releases, wait 2-3 mins. (It's important that you do NOT have any destination selected, just sit in orbit)
3) Once done with the 2-3 mins, select a destination and choose the tower. (If it says Requirements Not Met, you will need to switch some armor around, then select the tower again and join in
4) You will be able to fly into the tower allowing you to buy his items. DO NOT leave the tower for any reason until you brought any items otherwise you will be reset to level 0.
How to Prepare for Iron Banner:
1) Have over 100+ Legendary Marks to buy his items.
(The Iron Banner Vendor does not take in Glimmer!)
2) YOU MUST BE ON YOUR OWN (No Fireteams Allowed)