There are families who prefer to toss flowers into a grave (or place them atop the casket), since it's a gentler touch than throwing a handful of dirt atop the coffin. This practice occurs at the end of the graveside service.
Immediate Family First
The deceased's spouse or long-time partner is the first to toss flowers into the grave. This individual should be followed by the deceased's offspring, then the deceased's siblings and their spouses and children, followed by relatives, then friends and other associates.
Follow the Family's Lead
Follow the example the deceased's immediate family sets for tossing the flowers. Watch where the family members stand at the graveside, how they drop in the flowers and how long they spend in silence at the grave before departing.
Your best etiquette guideline for tossing flowers is practicing common courtesy and respect. For example, don't make a big, dramatic "show" of dropping flowers in the ground. If the person ahead of you is very emotional and taking a long time to leave the grave, patiently wait your turn.