I have alot of experience with this. First make an alcohol extraction hash oil. If you use 91% Iso and soak for about 7-10 days, the purity should be about 50-60%, meaning 500-600mg full spectrum cannabinoid content per gram of extract. Take oil and melt into food grade glycerin at the ratio of your choice. Most sodas available at dispensaries have 100mg full spectrum cannabinoid content. Now that you have your hash oil/glcerin tincture. Carbonate water using sodastream and add the dosage you want along with flavoring. This advice was given to me by the owner of a large Colorado thc-soda company and seems to be the best and easiest way to make soda at home. I have left alot of basics out like hash oil making and tincture ratios, but those are very easy to find. hope this helps.