I am not sure about the usage of infinitives in this sentence:
Finally, one of the accused confessed to have forged the director's signature on the report.
Could anyone explain correct usage?
You seem to have confused the to particle used in certain infinitive constructs with to used as a preposition.
In English, the phrasal verb confess to takes the -ing form of the verb as its complement, not a bare infinitive as you have used here.
This acts as the head of its own verb phrase (VP), and is effectively a noun. When you use an -ing form as a noun, it is sometimes called a gerund. Notice how you can swap in a real noun:
There is also a non-phrasal use of confess, as distinct from the phrasal verb confess to: