One of the boys is my cousin

When we start a sentence with 'one of the', we should note two things:
a) the noun after 'the' must be plural;
b) the verb used should be singular.

The following sentences should make the above points clear to readers:
1 One of the boys is my cousin. [boys - plural; is - singular]
2 One of his friends was here yesterday. [friends - plural; was - singular