Each boat is a little different. First and foremost, the condition of the sail must be taken into account. If old and baggy, it will tend to bind where the baggy part is and much care must be taken to stop doing what you are doing (in or out) immediately and reverse (several times if necessary). Then slowly work the sail through what would have become a jam. NEVER FORCE THINGS!
With a newish main this should not come into play, but if your main is older, start saving for a new one or have it recut by a sail maker.
With a good, flat main, some boats require the boom to be set somewhere around a 7 degree angle up from level. Ours doesn't. We keep a slight tention on the clew going in and often use the wind to help pull it out. We also set, reef, shake out reefs and furl the imrf on every point of sail, including ddw in light wind (just keep the boom amidships).
If you want to be one of us who love our imrf, then take it easy and remember don't force things. There is a learning curve and once you've mastered the system, you'll never want to go back to any other system.