In order to enter scores, you will need to publish your schedule.
You will see score input boxes on each game starting the day it is scheduled (see screenshot).
Simply input the scores and hit Save Scores on the bottom right.
Standings will update automatically.
Sub-Scores / Sets / Games
If your matches consist of multiple games, you can enable subscores (aka sets or games) in your standings configuration by turning on the track sets toggle. The free-to-use Volleyball template uses this by default, so you may use this as a starting point.
Once track sets is turned on, you can reveal the sub-score inputs for each game by clicking the + next to the match score inputs:
Remember, score inputs show up on the day the game is scheduled.
Once a match has a score, you can clear that score in its edit game dialog (click the little green icon above the team on the right).
in the edit game dialog, submit empty scores and 0 points (or leave points untouched – those just can't be blank):
Once a score is recorded, you can also change the points given to each team via the edit game dialog (for sports where points are a factor in standings, e.g. soccer, hockey).
You can designate other users as scorekeepers and let them submit scores: see this article.