## Steiner Triple System

I need to construct a Steiner Triple System of index 1 having 21 varieties.

I know we can take a Steiner triple system having 3 varieties and a Steiner system of 7 varieties then we can construct a steiner triple system of $3 * 7 =21$ varieties. However, I'm not sure how to go about constructing anything further. Any help would be appreciated.