In G₂ manifolds, 3-dimensional associative submanifolds (instantons) play a role similar to J-holomorphic curves in symplectic geometry. In [21], instantons in G₂ manifolds were constructed from regular J-holomorphic curves in coassociative submanifolds. In this exposition paper, after reviewing the background of G₂ geometry, we explain the main ingredients in the proofs in [21]. We also construct new examples of instantons.