Mezclo la harina con agua y luego añado la levadura.

In Spanish (including Spain), recipes and instructions are often written in the present tense to describe a sequence of actions as if the speaker is doing them: Mezclo… luego añado… = I mix… then I add….
You can absolutely use the imperative too, especially when directly telling someone what to do: Mezcla la harina con agua y luego añade la levadura. Both are natural; the present tense can feel more “recipe-narration” style.

Spanish normally drops subject pronouns because the verb ending already shows the subject. Mezclo already means (I) mix, so yo is usually unnecessary.
You might add yo for emphasis or contrast (e.g., Yo mezclo la harina y tú añades la levadura).

In Spanish, it’s very common to use the definite article with ingredients when talking about them in a specific step of a recipe: la harina, la levadura. It often sounds more natural than leaving the article out.
You can omit the article in some contexts (more like a general category), but in recipe instructions, la + ingredient is extremely typical.

With liquids/ingredients used as a substance, Spanish often uses no article: con agua = with water (some water).
con el agua would usually refer to specific water already identified in the situation (e.g., with the water from that bowl / the water we measured earlier). In recipes, con agua is the default.

Agua is feminine (el agua fría, un agua mineral) but it takes el/un in the singular to avoid the awkward double a sound (la a…).
In this sentence, there’s no article, so you don’t see that rule. If you did add one, it would be con el agua (but again, that would sound like a specific water).

Usually no: …con agua y luego añado… is fine without a comma.
You might see a comma if there’s a stronger pause or if luego starts a new clause more clearly: …con agua, y luego añado… (less common) or Luego, añado… (more common).