Process tailoring deals with customizing a standard software development process to meet an organization needs. It is typically based on the manipulation of an existing software process to incorporate new activities or remove existing ones. Therefore process tailoring must be constrained by a set of rules to guarantee the consistency of the resulting process. This paper presents a metamodel describing a set of elements, relationships and well- formedness rules required to ensure tailoring of standard software development process based on RUP process. We also present PRoTTo, a prototype developed to support the process tailoring based on the proposed metamodel and well-formedness rules.