Mix the syrup, sap, and yeast, and allow to ferment in a container such as a plastic pop bottle open for 2 days, or until the fermentation is visible, then seal, and refrigerate. When the container is visibly swelled from the fermentation, you can open the bottles and enjoy.
After 2-3 more days, crack open the container and try some, you can put the container back in the fridge, tightly sealed. Enjoy the soda within a few weeks, but make sure to check on it occasionally as the fermentation will continue and it can overflow if left too long.
Notes
Safety note on containers A glass container looks pretty, and is what I have pictured here in this post, but know that with making your own ferments, it can be dangerous if you don't keep a close eye on it in the fridge. I have had glass bottles explode before, and they can be dangerous. If you're just starting out, I highly recommend using a plastic pop bottle with a screw on lid since you can feel if it's pressurized. This is a soda meant to be drank straight from the bottle. If you want extra carbonation in your soda, you can cut it with unflavored soda water, if you want to do that, double the amount of syrup in the soda to 300 grams, or sweeten to your taste.