“Glacé” is a confusing term because it can refer to ice cream, cake frosting, fruit candied in “hard crack” syrup, or simply fruit preserved in syrup. It’s that last definition that applies here. Most sources I consulted had a similar procedure for making glacé cherries:
Make a simple syrup of one part water and one part sugar. Bring to a simmer, add pitted cherries, remove pot from heat, cover and let stand over night. This is simply to infuse the syrup with cherry, and the cherries with syrup. The next day, remove the cherries and reduce the syrup until a candy thermometer reads 230°F. This gives a good thick-but-runny consistency. Reintroduce the cherries, simmer briefly, then store in a sanitized jar.
I used to fill my Christmas fruitcake with glacé Bing cherries, but a few years ago I switched to our local Evans cherries instead. They were so soft after the glacé process I worried they would be too delicate to fold into the dense pound cake batter. While they definitely don’t hold their round shape like the bings, they managed to stay in one piece. Their tartness is a welcomed addition to the cake.
The syrup that the glacé cherries are preserved in is fantastic in sparkling water, or cola, or cola and rum.