The purpose of the modified clip is to maintain the standard Cocker look, while reducing grooming chores. Start by looking at the problem spots on your pet, and modifying his grooming accordingly. Your dog will be healthier and happier.
When I used to take my dogs into the groomer, I'd bring along a list of instructions. Not all groomers clip Cockers in the same manner. With a list, there is less room for confusion.
Let's start with the ears. Most of these steps should be included in any kind of clip, so I'm surprised how few groomers follow them. The basics here are to lighten the ear flaps and remove all hair from the ear opening. This allows more air to reach the ear canal and helps keep infections from getting a start. As with the eyes, allowing a discharge to build around the ear opening promotes bacteria growth.
All hair is clipped away from the ear canal, as short as possible. The underside of the ear flap is shaved short to reduce weight. Use some care when approaching the sides of the ear flap, to avoid clipping "patches" from the top fur. This will leave the finished ear looking "patchy". Use cool clippers and avoid clipping the fur too short on the underside of the flaps. 1/16 to 1/8" is best. Overly shortened fur will be itchy and your pet may scratch their ears raw.
On the outside, fur on the top 1/3 of the ear flap is cut short. On the lower portion, the fur can be left at a maintainable length. Remember, the longer the fur, the more often it will need to be combed. This should be a compromise between "style" and manageability. As old fur drops out, it will begin to tangle in the remaining fur, and this is especially true where the fur is curly, as in the outer ear flaps. For this reason I generally trim this fur to a length of 3 inches. Trimming this length can be time consuming. I generally comb the fur up, using the comb as a guide for scissors or clipper. For beginners it is easier to use a 1" guide on the clippers and clip downward. I all else fails, clip it to an even 1/2 to 3/4". At this length, the fur will be manageable for a year, as it grows back. Field dogs will do better with a shorter length, while indoor pets can have fur somewhat longer.
Fur on the sides of the head is cut moderately short, to a length of less than 1/2 inch. Again, use care to clip with cool clippers and leave enough fur to avoid itchiness. One of my pets has sensitive skin, and she will scratch her skin open if clipped too short. At the first sign of redness, I apply Neosporin. This reduces the itchiness and avoids infections.
Ticks like to attach themselves in the soft portions, while fleas migrate to warm, protected areas. Ticks will often attach themselves to the abdomen, while fleas migrate to tangles, the abdomen and ears. The abdomen can also difficult to keep clean, especially in field dogs. Tangles also tend to form under the Cocker's legs, due to constant movement. If these areas are cut short, these problems are minimized.
When clipping, I like to remove fur from my pets' arm pits (the area where the front legs meet the body). This eliminates most of the tangles that form there. Care must be used to avoid clipping too short and causing a razor burn here.
I clip the entire belly of my dogs, to a length of 1/8 inch, in the diaphragm, groin, and the inner thighs. When the dog is standing or sitting, the side feathering makes this clip unnoticeable. Fleas and ticks are easier to spot on a bare belly. The area stays cleaner, there's no hair to tangle, and your pet will be more comfortable in summer. Belly pets are also enjoyed more.
A traditional ACS cut calls for the feet to be left shaggy, which I don't understand. Cockers have very dainty feet that look quite attractive when clipped short. If you wish to maintain the traditional look, clip the feet short to the first joint, and a heavy feathering on the legs can be left to cover the feet. This is quite attractive, eliminates a lot of tracking, and helps maintain foot health. This also makes nail trimming much easier.
Trim the fur on front feet back to the first joint, to a length of 1/8". Use scissors or clippers to clip the hair from between the pads and take care to clip between the toes. The rear legs are usually trimmed short to the second joint (the knee).
Toenails should be kept short and rounded. Long nails are uncomfortable and can split or become snagged. When my dogs start to "tick" on the floors, I know it's time to trim nails. The nail is an interesting structure, growing similar to a bird's feathers. The hard outer cuticle grows around a center quick. As the nail grows outward, it extends beyond the quick. Long nails can begin to split. Splitting back into the quick is very painful. Careful trimming helps avoid this situation.
I use pliers type clippers and keep a sharp blade in it. Dull blades will tend to crush the nail, rather than cutting cleanly. Light colored dogs have transparent nails and you can see the quick. For dogs with black nails, I usually clip just past the wear pattern. Keep a jar of alum handy in case you clip the nail into the quick and bleeding starts. Dampen your finger, dip it into the alum, and apply. Bleeding will stop in a few seconds. Frequent trimming will cause the quick to pull back, allowing shorter clipping of the nails.
I also use a battery powered Dremel tool, with a sanding drum attached. This allows me to quickly grind the nail round after clipping. This eliminates sharp corners and allows me to get closer to the quick. The heat of grinding will cauterize the quick, and so bleeding doesn't occur. Most dogs don't like the sound of nail clippers, especially if the quick has been clipped once or twice. On the other hand, the Dremel doesn't invoke these reactions.