First, comb him out until he's fluffy. Second, shorten his hair by plucking (pulling) out the longer hairs. In other words, where the hair is too long, pull out the long hairs with your fingers (rubber finger tips will help) until only the shorter hairs are left. It comes out quite easily, and the hairs that grow in will be a little thicker and coarser and easier to take care of in the future. If you don't want to pull the hairs out with your fingers, then use the stripping comb (see grooming tools below) and rake the stripping comb through his body coat to pull out a lot of the hair. Some breeders only scissor their dogs, but I find that the hair is easier to maintain and has a better quality when at least some plucking is done. After plucking the body, bathe your Pumi. When finished bathing, pat your dog dry with a towel and let him air dry on his own. This will allow his naturally curly hair to curl back up again. Bathing with shampoo will soften the coat for a week or two, but the firmness will come back. When your Pumi is dry, use scissors and trim the body, head, neck, buttocks, forechest, and legs, according to the diagram (from the Hungarian Puli-Pumi-Mudi Club).
 
The dark areas on the diagram illustrate where the coat is at its shortest (about 1-3 cm or 1/2 - 1-1/4 inches ), the white areas represent those areas where the coat is at its longest (7 cm, 2-3/4 inches maximum), in the gray areas the coat is somewhere in between. The length of coat is measured when the coat is curled up, so the actual length of the hairs is longer than these measurements. The final length of the coat depends on the structure of the Pumi: the coat of a strong-bodied Pumi should be kept shorter, while a light-bodied Pumi can have longer hair, etc.  There shouldn’t be any clear edges in the coat - everything should be rounded. The front (forechest) and back (buttocks) of the Pumi are trimmed shorter to preserve the correct, square look. Be careful with the backline so that it doesn’t look arched, sway-backed or slant the wrong direction. On the head you should pay attention that the skull and the cheeks don’t look too massive in comparison with the muzzle, so the cheeks should be cut fairly short. The profile of the head should be straight (no stop).  The hair on the ears is rounded and trimmed. If the ears are heavy, you should cut more hair off and vice versa. It is good to cut more hair on the outside edges of the ears so that they look as well set as possible. If your Pumi's ears fold over too far, cut as much hair as possible off the ears so there is less weight holding them down. Also you can trim the hair on the top of the head shorter to make the ears appear higher. The opposite is also true, if the ears stand too far up, leave more hair on the ears to hold them down. Pull out the hair that grows inside the ear canal (my Pumi hates this, but it's better for their ear health). The hair on the tail is trimmed (1) and shortened so that the hair doesn’t "part" clearly.  You should be able to see daylight (2) through the inside of the curl in the tail. The hair on the legs usually doesn't need to be trimmed very much, but make sure the legs look straight and that the hair on the feet is rounded. Also be sure to cut and trim the nails.