The "Let's Build A Simple Interpreter" series of posts (starting at https://ruslanspivak.com/lsbasi-part1/ https://ruslanspivak.com/lsbasi-part1/) is excellent,

The "Let's Build A Simple Interpreter" series of posts (starting at https://ruslanspivak.com/lsbasi-part1/ https://ruslanspivak.com/lsbasi-part1/) is excellent, geared towards building an interpreter for a Pascal-like language in Python. It does not use a parser generator, and builds up everything incrementally. Unfortunately it's incomplete...