please help~~graph theory problem 3

Consider the following scenario. The National Rugby League is a sporting competition
involving 16 teams. Unfortunately, this year matches began before the schedule for the
whole season was agreed to. The following requirements are to met. Each weekend there are eight matches (with each match involving two teams) and each team plays exactly once each weekend.
In the season each pair of teams should play two matches against each other.
So far there have been 17 weekends of matches. Over the first 15 weekends, every pair of teams played one match against each other. On weekends 16 and 17 the following matches were played.

Weekend 16
Cowboys v Broncos
Storm v Warriors
Knights v Raiders
Eels v Panthers
Tigers v Roosters
Sea Eagles v Dragons
Sharks v Bulldogs
Titans v Rabbitohs

Weekend 17
Broncos v Storm
Roosters v Sea Eagles
Dragons v Sharks
Warriors v Knights
Raiders v Eels
Cowboys v Panthers
Bulldogs v Rabbitohs
Titans v Tigers

It is now coming up to the eighteenth weekend and your task is to design a schedule for the remaining matches of the season.

how did you go about solving this? i first used excel but stuffed it up so i thought about using graphs...is it going to take me forever? is there a mathematical way of solving it or just graphical?