This can depend on how you made the Elderflower Champagne, some people opt to bottle up just before the fermentation is complete, the last bit of fermentation is then in the bottles and creates the carbonation (although you have to be careful not to bottle too soon or the bottles may over-pressurise).
The other common option is to add a little sugar to each bottle, this then secondary ferments in the bottles a creates the fizz, it is a bit more of a controlled way of doing it as you can control the amount of sugar and therefore the level of carbonation, as a guide around a level to a heaped teaspoon per 500ml bottle.
If you find this current batch is not very fizzy and you would like it a bit more carbonated, some people just mix it with a little lemonade or similar just to make it more lively
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The other common option is to add a little sugar to each bottle, this then secondary ferments in the bottles a creates the fizz, it is a bit more of a controlled way of doing it as you can control the amount of sugar and therefore the level of carbonation, as a guide around a level to a heaped teaspoon per 500ml bottle.
If you find this current batch is not very fizzy and you would like it a bit more carbonated, some people just mix it with a little lemonade or similar just to make it more lively