1) Recall the grammar/rules of the output language (for both natural language and formal languages).
2) Form the idea of what it is you want to express in your head using your internal language (usually your first natural language).
3) Encode the idea you have in your mind using the formal rules from step 1.
Step 1 is usually internal and subconscious for natural languages that you can communicate in freely and to an extent this is even the case for other languages and even programming languages.
For your first natural language, step 1 is usually done behind the scenes by your brain automatically, or these rules are already present in your brain in a usable form, and step 3 is trivial because you are just expressing the idea which is already in this language in your mind.