This is a big question, with many parts to it. Cleaning can directly affect the Safety, Efficacy, Strength, Purity, and Quality of a product, and so is a topic of much debate and much scrutiny.
There is too much to this question to answer in one post, and this will take a systematic approach to learn all the aspects.
What do you know already? What do you have in place?
Here are some aspects of cleaning validation that will have to be delved into deeply.
1 - Toxicology of your product (helps determine acceptance limits, see Boomer Chemist’s comment)
2 - Equipment design (sites to test for)
3 - Cleaning method (automatic, or manual, or a combination of both)
4 - Cleaning agents, surfactants, etc.
5 - Chemistry of the product to be cleaned (how hard/easy is it to clean
6 - Interaction between various products (if the equipment is shared)
7 - Types of products to be cleaned (are there various “types” of products
8 - Bracketing approach (is there a worst case product, if there are multiple products)
9 - Batch or campaign cleaning, and how often cleaning needs to be done. How often sampling and testing need to be done.
10 - Sampling methods (swab or rinsate)
11 - Testing methods (TOC, HPLC, pH, or other).
Each of these bullet points deserves a full discussion. So your task is fairly large.
Many of these topics are found within this discussion forum. Here is a quick link to search.