Spaces - No Commas, No "Quote" Signs or other Special Characters
Single words must simply be separated by spaces.
dog house wood winter garden
Every Word Only Once
It is sufficient to only enter all unique words once. It has no effect to enter them twice or more - neither positive nor negative.
If the word is already within the product's title or bullet points, repeating it in the backend keywords is also not neccessary.
The keywords order has no influence.
It is perfectly fine to enter the words in lower case letters, although you may also choose to write them capitalized. It is of no relevance.
For example, "dog house" and "Dog House" will lead to the same results.
Include both Singular + Plural Versions & Filler Words
According to Amazon it is supposed to be enough to only enter one of the two versions and not necessary to enter filler words. In reality though this is often not the case. It is therefore recommended to simply enter both versions as well as any filling words to stay on the safe side.
For example, enter both "dog" as well as "dogs".
Also, enter words such as "for" or "and" at least once, to be sure you are found for searches like "house for dogs".
Enter as Many Relevant Keywords as Possible - Keep Limitation in Mind
In general all keywords related to the product should be added. For most product categories this is limited by 250 Bytes though - learn more about this here.