There are many ways to do it.
1. The original way (4 people - easy) Set a ness and falco to team1 and peach and fox to team 2. Go to any stage with two platforms with a space between them (Temple, BF, Yoshi Story, etc.) Give the ness an infinite super scope. Have Fox and Falco get on the platforms with peach under them in the middle. Stand Ness behind falco and have the space animals put up their shines. Ness now shoots the tiny balls repeatedly (going through falco's shine, into fox's and back to falco's). Do this until it looks like a consistant string of balls:
Yes:
OOOOOOOOOOOOOO
No:
O-OOO----OO--OOO-
Now have peach start throwing turnips up into the same area until 14 are in there. (I sugest the C-Stick so that you don't accidentally jump up into it) After that, have fox and falco drop their shines and there you go!
2. Ice climber way (3 man - hard) This one requires no teams, but a little more coordination. It does, however, give you more freedom of where you can do it. Find a place where falco can shine across from being shot at and peach can be under (this now can include Peach's castle, flatzone, etc. [just no moving stages like rainbow road]). Give the ice climbers, both of them, infinite super scopes. Have them stand like with the other one and start shooting. Except with this one, you are going to have to try to do it when the balls are not stringed together, but in the best possible spot.
O-O--OOO---O-O---OO--
............^here.....................
Have Peach do her thing and sooner than you think, you have a black hole.