From an idea to a server you actually use: picking the problem, choosing tool or resource, transports, tool schemas and error messages a model can act on.
An MCP server worth writing usually starts as an irritation. Something you look up three times a week, in a system with no usable public API, and every time you copy the answer into a chat window by hand. That is the whole test. If the irritation is real, the server takes an evening. If it is not, you are about to build a wrapper that nobody opens twice.
This is the build piece, not the explainer. What MCP is and why it exists is covered elsewhere, and so is which servers are worth installing before you write your own. What follows assumes the vocabulary and goes straight to the decisions: what to build, what shape to give it, how to write for a reader that is a language model, and the mistakes that produce a server which passes every test and gets uninstalled within a week.