Read the following sections of the Agda documentation:
Also have a first look at:
Fin and Vec started
in Friday's lecture, FinVec.lagda.md.
You will at least have to insert arguments or pattern matching
before working on the holes provided.
Use the interactive support that Agda gives you,
in particular case splitting `C-c C-c`, give `C-c C-SPACE`,
goal type and context `C-c C-,`, …