For example, which one of the following sentences can I use?
Consumer of Product X needs to fill out a rebate form […].
Consumers of Product X need to […].
A consumer of Product X needs to […].
This is for a general-purpose announcement.
Countable nouns need an article or similar, if singular:
A consumer will fill out a rebate form.
Each consumer will fill out a rebate form.
The consumer will fill out a rebate form.
My dog ate my homework.
Generics/uncountable nouns need to be used without:
Blood is running through the veins.
Water will be refilled at the next Oasis.
The second one is the best choice. Or even better, All consumers of product X need to [...].