What is a synonym for recipe?

Formal:

* Formula

* Instruction

* Method

* Procedure

* Guideline

Informal:

* Directions

* Instructions

* How-to

* Guide

More specific:

* Cookbook (for a collection of recipes)

* Formula (for a specific type of recipe, e.g. chemical formula)

* Guide (for a recipe that provides detailed instructions)

The best synonym will depend on the context of your writing.