You want to double your yeast count essentially.
a 2L starter will accomplish just that with intermittent shaking(when you walk by the area just give your starter a shake).
No fancy equipment needed. You can even do a starter in the pan that you're boiling the wort in.
The ratio couldn't be simpler. 1 gram of DME to 10 mL of water. In your case youre gonna want 200 grams of DME and a half gallon of water, you can also add an 1/8tsp or so of nutrient.
Just bring the starter wort to a boil, boil for 15 minutes cool to room temp pitch yeast and shake. Cover the top of your vessel lossely with aluminum foil. No airlock necessary, you want the starter to breathe and get oxygen intake. Your purpose here is yeast growth and health not drinkability.
I always recommend doing a starter in any beer over 1.040 SG, and even then I usually wake the yeast up with a tiny 500mL starter or so. If you are making the switch to liquid yeast then I would highly recommend investing in a stir plate and 2L erlenmeyer flask with a stir bar. www.stirstarters.com
is a great place to get all of this in one shot for about 75 clams. Pitching the appropriate amount of yeast and controlling ferm temps is the MOST crucial aspect to making better beer if you ask me. Hope this helps!