To enter a flow state you need to be working on something moderately challenging, but within your ability. It has to be a task that takes concentration and will take at least an hour to complete. You need to be relaxed and cannot have any distractions around you. It takes a good 15-20 minutes to get into a flow state.

For me flow is very pleasant but I don't get to experience it often, maybe once a month. It always feels very productive. I can be in a light flow state where I am still slightly aware of my surroundings, but on rare occasions I have experienced deep flow where I am aware of nothing but my internal monologue and the code. I daydream about being able to get into that mode sometimes.