Most people won't reccommend this as it is believed they aren't great to feed your fish. (If this is why you want to do this.) However, set up a little 3-5 gallon aquarium. Fill with Freshwater and let it cycle (just like salt). After it has cycled add fish. Then remove half the water and drip in (like acclimation except one drop ever 3 second, not 8 drops every second) salt water you have pre mixxed to double the strength of your current salt tank. As you can see this takes a longtime.
When it is done you aquarium salt level should be the same as your current saltwater tank. You have just created your guppy feeding tank.