Every part of a cattail can be used. The parts are seedheads, flower heads, young flower heads, leaves, young shoots, and rhizomes.
The seedheads can be used as insulation in clothing, tinder, or soft stuffing for pillows. In most situations you'd just use this for tinder. I expect to have enough clothes and pillows that I won't need to use the seedhead. If you do use the seedhead as stuffing make sure that the material holding it together is thick because you may get a reaction from it rubbing against your skin.
The mature flower can be used as flour. Put a plastic bag over the flower head and shake the flowerhead. The flour can be used as is or mixed with other flour. With a younger flowerhead that is turnning from green to yellow remove the husk then you can boil and eat the flowerhead.
Young sprouts that are about a foot and a half tall can be cut, peeled, and eaten raw or cooked. Once the plants get bigger than a foot and a half you don't eat them. At that point the only thing you can do with the leaves is use them for thatch for a shelter, or for weaving.
The rhizomes are starchy and can be used as potato substitutes. They can also be dried and ground into flour. Growing off the rhizomes are little roots. These can also be used by peeling away the outer husk and eating the inner core.
Having skills to feed yourself while on the go are important. It's not something that you should do for the first time during a stressful period. Find some cattails this weekend and try it.