The rules and a set of counters for Skaven teams in Blood Bowl first edition were published in White Dwarf issue #86 (Feb 1987).
Gameplay-wise, Skaven teams have faster players than most other teams, can't have Defensive or Kicking players and use a different Star Player Table, where the most common result is a Chaos mutation (a roll on the Mutations Table).
The rules also include an example of a veteran Skaven team, the Skaven Scramblers.