Article 51 — Administration of sacraments

III. WORSHIP, SACRAMENTS AND CEREMONIES
The sacraments shall be administered only in a church service by a minister of the Word with the use of the adopted Forms, and under the supervision of the elders.

This article is based on what we confess in Article 33 of the Belgic Confession of Faith, and Question and Answer 66 of the Heidelberg Catechism.
It only adds that the sacrament shall be administered in the midst of the congregation and by those who have been mandated to administer them by Christ, the Head of the Church.