You will need :

132 grams of Gamma Butyrolactone (GBL)
91 grams of Potassium Hydroxide (KOH)
  or 63 grams of Sodium Hydroxide (NaOH)
Stainless Steel, Corningwear or Pyrex saucepan of at least 1 Qt capacity
High Temperature Thermometer (recommended but not essential)
Pyrex measuring cup of at least 2 cup capacity
pH test strips ( I recommend using Macherey-Nagel pHfix 7-14 strips, pH paper can give false readings)

1) Dissolve 91 grams KOH or 62 grams of NaOH in 200 ml [distilled] water. Use a Pyrex measuring cup of at least 2 cup capacity

2) Place 132 grams (120 ml)of GBL and 50 ml [distilled] water in a stainless steel, corningwear or pyrex glass saucepan of at least 1 Qt capacity. Do not use aluminum cookware

3) Heat to Approximately 100° C (212° F) - (If you don't have a thermometer heat until the water at the bottom boils be carful not to burn it)

4)Remove from heat

5) SLOWLY add the KOH/NaOH solution into the GBL while stirring, adding small amounts (approx. 1-2 tsp) at a time every 3-5 seconds . Do not add the GBL to the NaOH, it must be the other way around!

5a) After adding 90% periodically measure the pH, continuing to add NaOH/KOH until the pH is between 10.0 and 10.5

6) Bring to a boil, remove from heat and stir 5 minutes.

7) Let it sit overnight (min 8 hours).

8) Measure the pH again. If you did everything correctly, you should now have a pH of between 8.0 and 9.5; So far I have gotten a pH in that range every time. However, if you made an error, it is correctable at this point:

8a) If the pH is above 10 add an acid (HCl, ascorbic acid, vinegar, lemon juice, etc.) to lower the pH to between 7.5 and 9

8b) If the pH is below 7.5, repeat steps 3-8 using a little more NaOH.

9) Dilute to desired concentration using boiling water.

9a) For Gamma-OH™ concentration: dilute to a total Volume of 970 ml

9a) For Xyrem™ concentration: dilute to a total Volume of 390 ml

10) Store in a sterile glass bottle and keep refrigerated.

11) Neutralize the remaining NaOH or KOH solution with acid (HCl, ascorbic acid, vinegar, lemon juice, etc.) before disposing.