Whatever works for you...

The cure zone page on The Master Cleanse says this:
//www.curezone.org/cleanse/bowel/master_cleanse.html
>>Use fresh lemons or limes only, never canned or frozen lemon juice. Use organic and vine ripened when possible. Also, Mix your lemonade fresh just before drinking. Don't mix it up in the morning for the whole day. You can, however, squeeze your lemons in the morning and measure out the 2 Tbls when needed. <<

Although I don't find it in the book one way or the other.

It makes sense not to pre-mix it though, because the Sugar in the syrup mixes with the lemon juice and starts fermentation. If you do pre-mix, make sure you don't make more than one days worth. I do like Starchild does and squeeze the lemons ahead of time, then mix as I go along.

I wish I could find my electric juicer, it would make it easier, but I did find my hand juicer and it helps a lot, you can find one at any cooking store for under $5. I find though that even after juicing them, if I squeeze the rind, I get more juice to come out...

Keith S. :{)>