Assuming you have a Sodastream machine, to make cola flavored seltzer water, you would first need to insert the carbonating bottle filled with cold water into the machine. Then screw on the Sodastream cola syrup cap until it locks into place. Press and hold down the big button on top of the Sodastream for about 2 seconds, until you hear a hissing noise. The soda is now carbonating. Once done, unscrew the cap and discard it. Pour your sparkling cola into a glass and enjoy!