You dont.
Use it at Snoopy's doghouse to knock the googles from the tree, beginning his World War I adventure.
Snoopy
After the football scene, pick up the football and go left. Use the football at Snoopy's dog house and it will knock his flying helmet down from the tree.
After you play the football scene with Lucy and Charlie Brown, pick up the football and use it at the doghouse. The helmet falls on Snoopy's head and the adventure begins.
Pick up the football and go left to Snoopy's doghouse. Use the football to knock the goggles out of the tree, beginning Snoopy's "Flying Ace" adventure.
Charlie Brown is on the sidewalk when you enter Great Pumpkin Island, but he spends much of the game in his back yard left of Main Street. Snoopy is at his dog house at the far left of the island. Your first task is in the pumpkin patch to the far right. You have to weigh the pumpkins, roll one back to Lucy's house, give Pigpen the bag, and give Linus the lemon sucker. Then you can go left where Charlie Brown is collecting the leaves with Snoopy.
Pick up the football and go left to Snoopy's doghouse. Use the football and it will knock the helmet down from the tree, starting Snoopy's Flying Ace adventure.
The Great Pumpkin
Snoopy fights the Red Baron .
go left on great pumpkin island
click on him
The mask you need is on a scarecrow after you complete Snoopy's Flying Ace adventure. There is no mask in the pumpkin patch. You can create a costumizer version (which does not help on the quest) by pressing the three keys Ctrl+Shift+P, and the same to take it off.