To create a petal rim to your cocktail glass mix the petals with some caster sugar. About 1/3rd petals to 2/3rds sugar.
Turn your glass upside down and carefully dip the rim of your cocktail glass in a shallow dish or saucer of sugar syrup or cordial (we like elderflower). Lift it out and hold it there for a moment or two to ensure that the syrup has dripped off and is not going to run down the glass when you right it. Then carefully dip the glass into the petal/sugar mixture so that it sticks to the glass. Shake off any surplus and right your glass.
If you are serving tequila then you could rub the rim of the glass with a squeeze of lime juice before dipping the glass in a mixture of salt and petals instead.