Jubilant Mascot — ruling 1

You choose the target creatures to support when Jubilant Mascot's triggered ability is put onto the stack, but you don't choose whether to pay {3}{W} until the ability resolves.

Jubilant Mascot — ruling 2

You can't put more than one +1/+1 counter on any one target using the support action.

Jubilant Mascot — ruling 3

Support can target a creature another player controls.

Jubilant Mascot — ruling 4

If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.

Jubilant Mascot — ruling 5

If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.

Jubilant Mascot : règles, fonctionnement et interactions (MTG)

Jubilant Mascot is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

At the beginning of combat on your turn, you may pay . If you do, support 2. (Put a +1/+1 counter on each of up to two other target creatures.)

How it works : Jubilant Mascot

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2018· Wizards of the Coast
You choose the target creatures to support when Jubilant Mascot's triggered ability is put onto the stack, but you don't choose whether to pay until the ability resolves.
No other card is named in this ruling.
#2June 8, 2018· Wizards of the Coast
You can't put more than one +1/+1 counter on any one target using the support action.
No other card is named in this ruling.
#3June 8, 2018· Wizards of the Coast
Support can target a creature another player controls.
No other card is named in this ruling.
#4June 8, 2018· Wizards of the Coast
If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.
No other card is named in this ruling.
#5June 8, 2018· Wizards of the Coast
If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.
No other card is named in this ruling.