Bring to a boil, then reduce to a simmer and cook it til those vegetables turn to mush. I cooked mine for about 1 1/2 hours. Strain out the vegetables and then strain the broth throw a sieve to get rid of any seeds and dirt from leeks. Store in freezer safe, leak-proof containers in the freezer.
I haven't tried it but you could probably do this in the crock-pot on high for 4 hours or low for 8-9 hours.