when we arrived in antigua there were two lines that extended outside the terminal itself, one labeled "immigration", the other labeled "transit". having read previous trip advisor posts, i knew to look for the shorter line for connecting flights. i asked a few officials if the transit line was the one for us if we are connecting to montserrat, and they told us yes. it was shorter but not moving much faster than the longer line. then out of the blue an airport worker asked us if we were waiting for a connecting flight and then ushered us inside. we saw that there were several lines inside and it wasn't clear what the different lines were for, some of which were very short and some were very long even though they all seemed to lead people to the same set of immigration stations. fortunately the one we were led to was very short and we got through with no problem in relatively short time. my advice: have one person in your party hold a spot in the transit line, then you work your way inside and find an official who is standing there directing people and say with a big smile and friendly tone of voice, "i'm going to montserrat on the ___ o'clock flight. which line should we be in??" being nice and outgoing and friendly gets you far in this culture where people are really friendly (except the rude tourists). note that you will need to fill out both the arriving and departing information (top and bottom sections) on your immigration card because you will be departing for Monserrat.
once you get through the immigration and customs you will end up outside the front of the airport where you will head over to the liat airline desks, one of which is designated for flymontserrat. after you get your "boarding pass", which looks like the claim ticket you get for your shirt at the cleaners, you go through security and into the gait waiting area. this was somewhat chaotic with lots of people standing around like they are waiting to get into a concert with general admission seating. they are waiting in front of the area that gets you to the gate area. there is no seating by the gates themselves. you should listen to the overhead announcements and head to the gate area when they call your flight. while you are waiting, buy some English Harbor 5 year aged rum at one of the stores. its Antiguan, inexpensive, and better than you will find easily on Montserrat.
in our case we were listening intently but didn't hear anything about our flight until they called us by name and we scrambled and cut passed lines in the gate area to get to our gate, and that was true even for the people on our flight who had been to montserrat several times. there are only 8 people on the flight so nobody seemed to mind letting us through.
btw, the flymontserrat website said that your carry-on could only weigh 6 lbs, which is less than a laptop and a couple books, but they didn't weigh the carry-on. nonetheless, i wouldn't bring anything bigger than a backpack unless you plan to check it. you will end up holding it on your lap during the flight. sit on the left side of the plane if you want to get a view of the island as you fly in.