How do I apply for burial in a national cemetery?

As GA said. Since my in-laws had already signed up for a plot, I called the National Cemetary that oversaw our little one to make arrangements for them.

You will need the person's discharge papers to show when he served. The Wars he may have served in. My Uncle served in WWII and Korea. If there is a spouse her/his info should be given too.* Then the plaque, which is ordered at time of passing of the first spouse to pass, can have both names on it. The National cemetery near me requires cremation but not all of them do.

*a family friend passed away who was the Vet. When the wife passed away later, the Cemetery had no record of the spouse. It was straighten out and she is buried with her husband but one less thing to worry about if the records are complete. I then checked to make sure my MIL was listed as FILs spouse, and she was.

Contact your local VA office, either at county or state level, or at a VA hospital.