How do we apply for Medicaid for our parents who are in assisted living?

Well, at 87 they should already be on Medicare.
Medicaid is for those with less than $2K in assets/savings.
If their income is depleted by the AL expense, the NY healthcare EPIC helps pay the Medicare Part D drug plan premiums for members with income up to $23,000 if single or $29,000 if married. Higher income members are required to pay their own Part D premiums but EPIC provides premium assistance by lowering their EPIC deductible.

They said to try Medicare/Medicaid.

OK check the NY health care website. nystateofhealth.ny.gov

Only want Medicaid assistance for the supplemental medical insurance, not for AL.

Wait! Read your contract, many assisted living places do NOT take Medicaid. AND their insurance coverage should be converted to an institutional type coverage which may cost less. Ask your insurance company.
If your father was a wartime veteran, look into Aid & Attendance from the VA.
Usually an AL insists you show sufficient assets to be able to pay out of pocket for two years before you need Medicaid.