Double distillation of you neutral spirit is the most important method you can use to improve the quality and quantity of your finished alcohol.
1. Distil your wash as normal, you do not have to discard anything as you will do it on the second distillation anyway. This is called a "stripping" run because you "strip" the alcohol.
2. Collect the spirit and dilute to 45% using the calculator found here
3. For every 8-10L spirit add 1 tablespoon of sodium carbonate (not bicarbonate). This is sold as washing soda at the supermarket.
4. Run the still as normal, discarding the first 50-100mLs. This is called the "spirit run"
5. Dilute to 40% and carbon filter
To save time you can strip the alcohol from two or three ferments and then do a single spirit run.