Just pretend you're back at home helping Mom in the kitchen. Fill the hole and tamp lightly. Insert a fork. There are some areas where burying your brownie is not a good idea. The bacteria that normally decompose our brownies are not very plentiful in high mountain or colder regions.
Some areas have no soil, or soil is hard to find, as in the snow.
Some areas are so popular and heavily visited that burying your waste is discouraged. In these instances, it is better to pack your brownie and take it with you at least to a location where it can be buried. Special sealable containers have been designed to perform this function. Some people simply use another handy container from home, such as a Tupperware container. A length of PVC pipe with a plug in one end and a removable cap on the other also works well and is easy to pack.