The longer argument is covered in Do You Really Need a Snorkel for Scuba Diving?
If you are going to ignore that advice, or you are taking a class or diving somewhere that requires you to carry one, you should at least know how to use it properly. A snorkel is a simple piece of equipment, but using one comfortably depends on positioning it correctly, establishing buoyancy before you start swimming, clearing it without inhaling water, and knowing when conditions have made the regulator a better choice.
What a Snorkel Is Actually For
A snorkel allows you to breathe atmospheric air while your face is in the water at the surface. That is its entire job in scuba diving. It does not improve buoyancy, make rough water safer, or replace a regulator when you may be submerged.
Its useful operating environment is therefore fairly narrow. You need to be at the surface, you need to want to remain face-down, and the water needs to be calm enough that repeated flooding does not turn every breath into a clearing exercise. If those conditions exist, a snorkel can let you swim without using gas from the cylinder.
If you are floating on your back, you do not need one because your mouth and nose are already in the air. If you are in rough enough water that you want a breathing source that will continue working when your face goes underwater, you already have a regulator designed for exactly that.
Set It Up So It Does Not Fight Your Mask
A snorkel mounted permanently to the mask is usually worn on the left side because the primary regulator hose normally approaches from the right. The exact mounting point matters less than making sure the mouthpiece reaches your mouth without pulling sideways on the mask strap or forcing you to turn your head.
Before the dive, put on the mask and snorkel together and actually place the mouthpiece in your mouth. If the snorkel is so high, low, or far back that you have to pull against it, move the keeper. A poorly positioned snorkel can tug continuously on the mask and make a perfectly good mask feel as though it does not fit.
Keep the snorkel as close to the side of the head as practical without creating tension on the mask. A large rigid snorkel sticking far away from your head is easier to catch on something and produces more leverage when it gets bumped. If you use a folding snorkel carried in a pocket instead, make sure you can retrieve and deploy it without turning a simple surface swim into another equipment-management exercise.
Get Buoyant Before You Put Your Face Down
Establish comfortable positive buoyancy before switching from the regulator to the snorkel. The BCD should be supporting you well enough that you do not have to kick merely to stay at the surface, because the fins should be propelling you forward rather than doing double duty as flotation.
The waterline should normally sit around your chin when you are relaxed in reasonably flat water. Once you roll face-down, your body should settle horizontally without requiring you to hold your head up or scull with your hands.
Trying to use a snorkel while you are still sorting out your buoyancy makes the skill unnecessarily difficult. If you are already breathing hard, struggling to stay up, or dealing with another problem, leave the regulator in and solve the larger problem first.
Switching From the Regulator to the Snorkel
Make sure you are stable at the surface before removing the regulator. Put the snorkel mouthpiece in your mouth, establish a comfortable seal, and take the first breath deliberately rather than immediately trying to breathe as though nothing changed.
If the snorkel is dry, breathing should feel straightforward. If there is any water in the tube, exhale through it first before taking a full inhalation. The mistake that causes trouble is assuming the tube is clear and taking a large breath through a snorkel that has partially flooded.
Your regulator should remain immediately available. A snorkel is being used because the conditions make it convenient, not because you have somehow committed to using it for the rest of the surface swim.
Clearing a Flooded Snorkel
A wave, splash, or poor head position will eventually put water into a snorkel. Clearing it should be routine if you intend to use one.
Keep the mouthpiece in your mouth and exhale sharply enough to drive the water out through the top or purge valve, depending on the design. The next inhalation should begin cautiously so that you can tell whether the snorkel cleared completely before taking a large breath.
If it does not clear cleanly, raise your head, breathe normally, or put the regulator back in. There is no reason to repeatedly fight a flooded snorkel at the surface just because you started the swim using one.
Dry and semi-dry snorkels can reduce how much water enters from splashes, but none should be treated as a regulator. A wave can still overwhelm the design, a valve can stick, and the inlet can still end up underwater.
Swimming With a Snorkel
Once you are buoyant and breathing comfortably, let the body lie horizontally and use a relaxed fin kick. Keeping the face in the water reduces the need to hold the head and shoulders up, which is one reason a snorkel can work well for someone who prefers face-down surface swimming.
You still have to navigate. Looking straight down while swimming is an efficient way to end up somewhere other than where you intended, so periodically lift your eyes or head enough to check your destination. Correct the heading and then return to a relaxed position rather than swimming the entire distance with your head craned upward.
Use a pace that you can maintain without breathing hard through the snorkel. The tube adds some breathing resistance and dead space compared with simply breathing atmospheric air normally, and there is no benefit in turning a surface swim into strenuous exercise.
When to Stop Using It
Repeated flooding is enough reason to switch back to the regulator. So are increasing chop, waves washing across the face, significant fatigue, difficulty clearing the snorkel, or any situation where you are no longer confident that the next breath will be dry.
I would make that switch early rather than after swallowing water several times. The regulator is already in the system, already works when submerged, and does not care whether a wave crosses your face.
If conditions are calm and you simply become tired of swimming face-down, rolling onto your back is another option. With appropriate BCD buoyancy you can breathe normally, rest, and continue the swim without using either the snorkel or cylinder gas.
Practice Before Depending on It
If you intend to carry a snorkel, use it enough that clearing and switching between it and the regulator are familiar. A calm pool or easy open-water session is a better place to discover that the keeper is positioned badly or that a particular snorkel is awkward to clear than a long shore swim before a dive.
Practice establishing buoyancy first, swimming several minutes face-down, clearing the snorkel after deliberately flooding it, checking direction without losing your rhythm, and switching smoothly back to the regulator. None of those actions is difficult, but familiarity keeps small annoyances from becoming distracting.
I still would not choose to make a snorkel part of my normal scuba configuration. If you do, use it for the narrow job it actually performs, keep it out of the way when you are not using it, and switch to the regulator as soon as conditions stop making the snorkel the easier option.