Boil water & sugar together. Once it boils, lower the flame & allow it to heat for another 1 minute. Remove from heat & cool completely. It can be made in advance & stored in refrigerator.
Use ⅓ cup of syrup to soak one layer of cake. If you are soaking White Cake then reduce the syrup per layer i.e. ¼ cup sugar in ¾ cup water. And soak each layer in ¼ cup syrup. With ⅓ cup syrup, I have found the cake to get over soaked.
for infusing flavor
If you are infusing flavor like rose petals, then boil petals once sugar is dissolved for 5-10 minutes. Cool & strain. Continue to use for soaking and assembling.
Notes
The more you soak the layers before filling, the more moist your cake will be.
You can infuse this syrup with flavors of your choice. I have used it with dried rose cake by infusing it with 1 cup dried rose petals.