Welcome, Jan123 -
Yes, the mixing bottle has a couple of "things" - it should be just about the size of the amount of liquid you're putting into it - and you do want the gas to go over to the receiving bottle. Jim's newest video is now showing a little venting hole in the nipple -
One thing about it, with the lid not tight enough, you didn't have a gas buildup in the bottle so high that the bottle "exploded" on you <G>. The use for the nipple on the baby bottle, of course, is to provide something that could come out easily if there was a gas buildup.
Next time, you might try having your stove ventilation hood fan running, pulling the gas up and out - there will always be a little leaking, because your receiving bottle has no cap on it at all. (And shouldn't) -
If it's any encouragement, I think most of us have had the "oopsy, too much gas" syndrome at one point or another. We usually only do it once.