• Hold stick in offhand, marshmallow in mainhand
• Use marshmallow to combine with stick into marshmallow-on-stick in mainhand
• Stand near the campfire, looking at it and wait until marshmallow is roasted
• Use roasted-marshmallow-on-stick to separate stick back into offhand and roasted marshmallow in mainhand
• Eat roasted marshmallow, gain short speed boost
• Repeat

Careful not to burn them!

This was completed in 1.12.2-1.5.0