Visual Languages for Human-Computer Interaction (HCI) are a good basis for the development of software environments to be used by professional people such as engineers, geologists, medical doctors, etc. The research we have conducted, along with the experience we have gained working with these people, has compelled us to develop software environments that not only are easier to learn and use, but also allow their users to tailor them to their own needs, and to even create or modify software artifacts. The design methodology that we have developed is refined in this paper, giving insights emerging from our recent experiences, and emphasizing how it supports human centric-computing and end-user development.