When you send for a cancel, you need to get a stamped envelope or postcard to another location and then get it back. You can do this in several ways.
The instructions for First Day of Issue postmarks are somewhat convoluted and subject to interpretation. I had to type this out from a printed piece of paper received from Stamp Fulfillment Services, because (as of this writing) the fully-updated instructions are not available on the Web.
Here they are:
POM Issue 9, July 2002
Updated With Postal Bulletin Revisions Through December 24, 2015
Customers may buy first day of sale stamps at their local Post Office; affix the stamps to their own envelopes, postcards, or other items described in section 231.63; and mail the items under separate cover to a participating postmaster/Post Office for First Day of Issue postmarking. All requests must include a self-addressed postage-paid envelope to return the FDOI postmarked items. All such materials must bear un-canceled postage, which includes the new first day of sale stamp, at the applicable First-Class Mail rate. This postmark is provided ony for philatelic purposes and should not be used to postmark bill payments, tax returns, applications, and/or other date sensitive mail. The items with FDOI postmarks applied must be returned under protective cover.
In practice, it works like this:
If you are sending for postmarks other than First Day of Issue, read on.
This is the cheapest method, but your philatelic collectible will have ugly spray-on cancels from automated processing. If at all possible, avoid this option. That leaves returning your cover or card inside another protective envelope.
This has a lot of merit to it, the primary one being that you only have to buy one envelope size. If you go this route, I recommend greeting card envelopes (sometimes called "A9 size"). Make a self-addressed stamped envelope from one of the A9 envelopes and fold it in half. Place it, along with a note to the postmaster and the materials you want canceled, inside another A9 envelope. Weigh to see if it's more than one ounce (and would therefore require more postage) and mail.
This is fine, especially if you're just starting out. The drawbacks of this technique are
This is the technique I use exclusively. Stiffen an A8 envelope with three or four 5×8 inch index cards (held together by adhesive labels if you like). Address it to yourself. Add postage. Place unfolded into an A9 (greeting card) envelope along with a note to the postmaster and the item(s) you want canceled. Your collectible(s) will be returned to you inside of the A8 envelope.
Whichever technique you use, be sure to weigh both the outgoing mailpiece (to determine if it requires additional postage) and the incoming mailpiece. This is often overlooked. You don't want your postal collectible to show up with postage due.
If you are sending away for a first-day-of-issue postmark or pictorial cancellation, the Postal Bulletin will give you detailed instructions, but they follow this pattern:
Quilled Paper Heart Stamp
401 Tom Landry Freeway, Rm 645
Dallas, TX 75260-9998
Highland County Historical Society Museum Station
301 North High Street
Hillsboro, OH 45133-9998
34 Walden St
Concord, MA 01742-9998
Let's say Jane Smith is looking for a Concord, MA postmark. Her letter should look something like this.
That's all you really need. If you are sending for a pictorial postmark and want to be super-sure, paste the sample image from the Postal Bulletin into your letter and say "I am writing to request this postmark". If you have a preference of where you want the cancel to strike, you can do a mockup of that, and the postmaster might be accommodating. But remember that these are courtesies extended by the USPS — especially in town cancel requests — and you should be gracious and polite in your requests and patient in waiting for your returns.