I just finished a lime extract for a Weiss beer, which is conditioning right now. I made the lime extract from about 18 limes. The green part of the peel contains the oils which I wanted and I also wanted the juice, but not the citric acid.
In the peel jar, I added Everclear, which worked in just a few days. Once the liquid is a dark green, heat the jar up to about 170 to evaporate the alcohol. I did this about 4 or 5 times just to make sure.
In the juice jar, I added some sodium bicarbonate (baking soda) to reduce the citric acid. I had to play around to figure out a good amount. I think I used about a 1/2 to 3/4 tsp for about 16 oz. After a week to a week and a half in the fridge, the liquid separates from the pulp and the sodium bicarbonate, which sink to the bottom.
After primary was complete, poured both jars into empty secondary and racked beer onto the extract. Overall, I used about 20 oz or so of both juice & peel extract. Turned out great!