We muck our chicken coop twice a year, spring and fall. This fall, we decided to buy extra straw to provide a ground cover in the chicken yard, which has become dead, compressed earth. Mucking is pretty straightforward--clean the poo out of the chicken coop, rake the yard, and lay fresh straw down. The chickens love to scratch in the fresh straw, so all we had to do is distribute flakes of the stuff around the pen and the girls took it from there.
Mucking is also a good time to make other chicken improvements. Tristan added more roosting space and attached the roost securely to the walls of the coop. With more chickens, they needed the extra space and strength to prevent them from roosting in the nesting boxes (which leads to poopy eggs) and to keep them from knocking the roost to the floor under their additional weight.