Genesis 8:20

And Noah builded an altar unto the LORD; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.

1. Noah built an altar in order to make burnt offerings to the LORD.
2. Considering the occasion, the burnt offering to the LORD appears to have been done either out of gratitude for the Lord's blessings or to symbolize their determination to do a better job of following him than those who had drowned in the flood.
3. Noah offered one of every clean beast upon the altar.
4. This reduced the number of each type of clean beast from seven to six.
5. From the plural "offerings" it appears each clean beast was offered separately. Whether each was done on a separate occasion or one right after the other is not clear.
6. This is the second time offerings are mentioned in Genesis, the first associated with the offerings of Cain and Abel. It seems unreasonable to think that the Lord would not have provided instruction to Adam concerning the type, method and occasions for these offerings, but nothing about this is mentioned.