Whenever you complete an Almanac by collecting all four pages, you earn a new crafting recipe for a Ben Franklin invention.
To craft an invention, use the accounting book - located in any general store or on the first floor of your manor - and go to the Crafting menu. Scroll down to the Inventions category and you will see all of the invention recipes that you have. Each invention requires specific artisans at a cetain level (who settle during homestead missions, and also level up by doing more homestead missions) and some amount of raw materials that can either be purchased in the Stockpile menu or crafted out of other materials.
The first one that I was able to create was the Kite & Key, which requires your tailor to be level 2. It also requires the linsey-woolsey material, which is created from flax. You are able to purchase flax from your farmers once you complete their pig-herding homestead mission.
You will eventually need to craft all of the Franklin inventions to get to 100% completion in the game.