Growing mimosa from seed takes some attention along the way so let me give you some details of the best procedure.

Even hardy types of mimosa are only moderately tolerant of the weather conditions in the New York City area so hopefully this plant is one that you have had success growing outdoors and the recipient will be able to plant it in a sheltered area for best results.  The biggest risk to health and survival of the plant will be inadequate water, poor soil, sudden temperature drops and alternating periods of hot and frozen weather.

In the late fall, when your seed pod is dark brown, it is ready to be cut from the tree with shears and opened to extract the flat, dark seeds.  You mentioned that the pods are now a rust color and they probably need to get a bit darker, to a true brown.  Place these seeds in a paper bag (not plastic), and tie the top of the bag shut.  Place the bag in a spot that is both cool and dark.  (Refer to the NYBG guide Collecting and Storing Seeds for information on the importance of seed storing conditions). 

In the spring, after the last frost, scarify (file or scratch the surface of) your seeds until pale spots are evident on each of them and then soak them in water for 24 hours up to a week if the seed is particularly large.  Plant the seeds in containers, in a mixture of equal parts peat moss, compost and potting soil, and cover the pots with plastic bags for humidity.  Keep indoors, in a sunny spot, until the plant has three set of leaves and is five inches high, watering regularly. Transplant seedlings into a mixture of sand and potting soil and keep indoors, in sun and uncovered, watering when the top two inches of soil are dry.  After three weeks, fertilize with a half strength, balanced fertilizer.  Once the seedlings are 12 inches high, they can be planted outdoors.

You are undoubtedly aware that mimosa trees have many seed pods and may self-seed in their new location.  The recipients should be aware that unwanted mimosa seedlings will have to be removed. 

Courtesy of the NYBG Plant Information Service