For fabric collars — you can wash our 100% cotton fabric-based collars in the washing machine on the delicate cycle, or handwash (though we strongly recommend that if you wash in a machine, please put them in one of those zippered garment bags (designed to protect delicate items like lingerie, etc.), and wash them completely in a cycle on their own (not with other laundry). If cleaning a fabric collar with stubborn stains, you can gently scrub the problem area with an old toothbrush. (Do this for fabric collars only / not velvet. See instructions below for velvets and other types).
For velvet collars — the instructions are similar to the ones above for fabric, EXCEPT we do NOT recommend any scrubbing or aggressive handling that may create bald spots in the pile of the velvet. This means handwashing may not be the best method unless you are purely dipping in soapy water and rinsing (not rubbing the collar in any focused manner). You can wash your velvet collar in the washing machine on the delicate cycle (though we strongly recommend that if you wash in a machine, please put the collar(s) in one of those zippered garment bags (designed to protect delicate items like lingerie, etc.), and wash completely in a cycle on their own (not with other laundry).
The next steps apply to both collar types mentioned above — Be sure to remove the bell and/or any tags that may be attached, beforehand. Normal detergent or even dish soap is fine. There is no guarantee that washed collars will look dramatically new if you've had for them for a while and they've taken on some serious wear, but they should look a bit cleaner after being washed. Lay them flat to dry and be sure to towel off any extra water on the metal parts to avoid rust.
For non-fabric collars (i.e. printed ribbon) — we do not recommend attempting to wash these due to the possibility of the ink reacting to whatever cleaning agent may be used. In some cases, the ink has been known to run when soap is applied.
TIP: If you are unsure about the material used for your collar, you can run a preliminary 'wash test' on a small interior (not outward-facing) section of the collar by applying some water and a little bit of detergent.
Tip for dealing with frayed bits — to keep collar collars looking tidy if they start to fray over time (which is inevitable given that cats have claws) — you can easily smooth away frays by quickly running the flame of a clicker "fireplace lighter" across any fuzzy bits and it will generally melt them away. (Do this when the collar is NOT on the cat).
Note: Applying the flame to a collar would only be effective in situations in which the nylon webbing base layer of the collar is starting to 'fuzz', which is naturally going to happen after extended wear. This will NOT work for snagged cotton fabric though -- only synthetic fibers, like nylon & polyester.