I have a T's on the thru hulls for head, AC, Yanmar, and generator. One side of T has a nipple and goes to the original hose. The other side goes to a valve with a nipple. I put a hose on the nipple and put it in a 5 gallon pail with salt-away mixture. I just started using the saltaway before just did a fresh water flush. Some people like to do a 24 hr vinegar soak to kill any marine growth. I've never tried that. I just do the flush at the end of a season or if we are going to be in a slip for a month I'll flush the generator. All our thru hulls are together at the bottom of the companionway steps so very easy to do this.
An added advantage I'm told is the thru hulls could be closed in an emergency and the engines used to assist the bilge pumps. Hopefully I'll never find out if this is true!
Bob