When I get around to writing the tutorial on MMSTD's (Mills Mess State Transition Diagrams) I'll probably start by explaining that Mills Mess is simply a shape distortion of a Cascade. In Mills Mess, all the throws are to the same height, the two hands throw alternately & every throw is caught by the opposite hand to the one that threw it, just like in the cascade. Unlike the cascade though the arms cross & uncross to make a pretty pattern. Siteswaps are also a distortion of the cascade. In 'Vanilla' siteswaps the hands alternately throw one ball at a time, the heights of the throws are varied so that on every beat there is a throw (or a hold). It follows then that you can apply the arm movements to a siteswap to get a new juggleable pattern.
Technically there are three different versions of 441 mess, they can be thought of as 441, 414 & 144. This tutorial covers the easiest version.
It will help if you can already juggle Mills Mess & being able to juggle 441 might give you a bit of an understanding of the pattern but neither are necessary requirements to learn this trick.
Start with all three balls, hold two in one hand & one in the other, cross your arms so that the hand with two balls is on the bottom.
Start by throwing one ball from the lower arm straight up to just above head height, keep your arms crossed over, as the first ball peaks, throw the ball from the arm on the top straight up on the other side to the same height. Both throws are made vertically, don't let them wander into the centre of the pattern.
After releasing the second throw quickly uncross your arms. As soon as both arms are uncrossed in front of you, pass the third ball being held in the hand that was on the bottom into the hand that was on top. Now recross your arms & catch the two balls one after the other. The first ball needs to be caught by the hand on the top; the second ball should fall into the hand on the bottom (the one holding the third ball after the pass. You should finish in the mirror position of how you started, with your arms crossed the other way around & with two balls in the opposite hand.
Now do the same starting from the mirror position. It may take some time to get this step smooth. One problem will be throwing the second ball too far inwards instead of straight up, especially if you have already learnt Mills Mess, you will probably throw a back toss out of habit. Just keep practising until you break that habit, say to yourself what you want the ball to do & focus on the spot where you want the ball to peak. Also make sure you throw the second ball then uncross your arms. If you release the ball while uncrossing your arms the ball will be carried in the same direction as your arm.
The trickiest bit of the exercise is making the pass in time. It has to be done very quickly & here's how I do it. As soon as you have release the second throw, uncross your hands (note that the ball starts off in the lower hand), then immediately recross your hands the other way. As your empty hand begins to pass under the other one, turn the hand holding the ball over so that the ball drops straight into the empty hand.
Keep practising this step from side to side, say to yourself as you do so, "Throw, throw, pass, catch, catch, stop. Throw, throw, pass, catch, catch, stop." Make sure the two throws peak at the same height & travel straight up & down in columns. To keep the full pattern under control you need to throw nice even throws & have a very quick & smooth pass in both directions.
When you can do that consistently starting from both sides it is time to try the full pattern.
Start as you did before, make the two vertical throws, then pass the third ball underneath. But instead of finishing by catching two balls in the lower hand, throw the passed ball straight up on the outside of the second throw. You are now back in the starting position, but after the first throw. Catch the second ball, throw the ball in the other hand straight up, uncross & recross your arms while passing the ball & do the same again on the other side.