From the Selected Edges.
- Separated two opposite edges
if you want to fill a space between two separate opposite edges, you should select the two edges with the Edge tool and then click the fill button in the menu panel.
Then the space between the two edges will be filled as upper picture.
- Connected more than two edges.
You can fill a space surrounded by multiple edges as the following image. With the help of the Loop Selection tool, you can select multiple edges surrounding a space easily.
After selecting edges connecting each other, click the "Fill" button in the menu panel then the space will be filled.
In order to fill a space looking like a polygon consisting of n edges, you can choose n-1 edges as well as n edges.