...
As can be seen, it is not simply a matter of editing an archetype and putting it into your hospital information system. The wiki page on Development and Governance of Knowledge Artefacts gives more details.
How do I develop a new archetype?
You can download the Archetype Editor and build various archetype-authoring tools to build one.
But if you are serious, you are probably a clinical professional, and will want to discuss requirements and ideas first with others, which you can do on the openEHR clinical mailing list, and also with others registered on the openEHR CKM.
...
CEN TC/251 has adopted the openEHR archetype model Archetype Object Model 1.4 and ADL 1.4 as the means of expressing archetypes to be used in conjunction with EHR data sent as CEN EN13606 EHR Extracts; these specifications are snapshotted in the revised EN13606 part 2. Archetypes created directly based on the EN13606 part 1 model fall into the category of legacy archetypes as defined above.
The openEHR ADL 1.5 and AOM 1.5 specifications, currently in late draft, will supersede the 1.4 specifications used by CEN and ISO 13606.