A key thing to pay attention to when creating a pronoun is to decide if the pronoun should be shown as a singular pronoun or a plural pronoun. Once you decide that a pronoun should be plural, you need to figure out the plural form of the pronoun. For some types of pronouns it takes more than just adding an "s" to make a plural pronoun.
The basic rule is that subjects and verbs have to agree in a sentence according to whether they are singular or plural. Plural verbs need plural subjects. This is also true of pronouns that act as subjects.
For some types of pronouns adding an "s" is enough to turn a singular pronoun into a plural pronoun. For some pronouns a different word may be required to show the plural form.
Following are guides of plural pronoun usage according to the type of pronoun:
Now that you understand plural pronoun usage, you need to add to that knowledge how to make antecedents and pronouns agree.
In some sentences, and even between two sentences, there is a pronoun that refers to a noun, pronoun, clause, or phrase that precedes it. The word, clause, or phrase that comes first is the antecedent and the pronoun has to agree with it.
Sounds easy enough, but there are some troublesome words when making pronouns and antecedents agree.
Other problems can arise if there are several words or phrases between the pronoun and its antecedent. Sometimes, a pronoun's antecedent is in the preceding sentence. In this case, just overlook the words between and that will help you determine the form needed.
Indefinite pronouns are the real troublemakers. These are singular and need singular pronouns: anyone, no one, everyone, someone, anybody, nobody, everybody, somebody, anything, everything, nothing, something, one, each, either, and neither. A word like "everyone" is treating people as a group, so it is singular. When using "either" or "neither", it is one or the other, so you are looking at one at a time, so those two words are singular.
When the antecedent is an indefinite pronoun (some, none, all, most, or any) that is followed by a prepositional phrase, agreement depends on the type of object. If the object is countable, the pronoun will be plural. Here is an example, " Most of the pennies fell out of their wrapper." "Pennies" is plural, so "their" is plural. If the object is uncountable (like sugar, butter, air, money, or furniture), the pronoun needs to be singular.
Start by deciding the type of pronoun, check the antecedent rules and decide on the plural form at that point. Practice will make it easier to understand and use the rules of plural pronouns.