you can do this from the start and you just simply move your player over the shape and you eat it.

you cannot do this from the start but it is fairly simple to get to it. You eat lots of shapes, grow larger and then you can eat the players that are smaller than you.

eat lots of yummy shapes while trying not to be eaten.

double click LMB to split into two when you have enough mass. This is good because it can
a) give you a speed boost
or
b) make you fast enough to catch and eat a player that is escaping you.

click RMB to shoot out some of your mass. this is useful if you
a) want a speed boost
or
b)want to play the friendly player and want to feed other players.