Place the glycerin soap and lemon zest in a microwave safe container preferably a large glass measuring cup. Microwave for 30 seconds, then check and stir with a chopstick or spoon. Remove when soap is just melted or very close to fully melted and stir.
Allow to cool while stirring. This helps the essential oils not dissipate off to quickly while being mixed in, but don’t be to fussy about it. Some will dissipate, but much will remain and the flashpoint of lemon essential oil is lower than the melt point of the soap base so you can’t stop it completely. Fragrance oils generally are less likely to waft away, but don’t have the benefits of essential oil either.
Pour the soap mixture into the silicone molds and set aside to cool.
Once full cool, pop the soap bars out. Be sure to store them in an airtight container or package after to avoid loosing the lemon scent.