Note that in most jurisdictions, a memorandum won't have any legal effect without a valid will that mentions the memorandum. In some jurisdictions, a memorandum won't have any legal effect, no matter what. It's important to find out from a qualified professional in your jurisdiction, whether a memorandum is enforceable and if so, what formalities there are.
Once you've confirmed that a memorandum will work in your jurisdiction, and you've agreed with your attorney that you'd rather distribute your stash by way of a memorandum instead of directly in the will, you need to actually prepare a memorandum. Check with the lawyer to see if there are any formalities, other than 1) referring to it in your will, 2) signing it and 3) dating it. If so, make sure to follow those instructions, in addition to the format suggested here.
Assuming your lawyer has approved of the use of a memorandum, here's an example of the sort of language you might use at the beginning of the memorandum:
MEMORANDUM OF PERSONAL ASSET DISTRIBUTION
Pursuant to the provision in my will for a memorandum to distribute my tangible personal assets, I direct my personal representative [or "executor" in jurisdictions that use that term] to distribute the following items to the named beneficiaries, provided that the named beneficiary survives me by thirty days.
After the introduction, for the description of the items, you can use either the general language that might otherwise have been included in a will (e.g., "all of my yarn" or "all of my wool yarn" to one person and "all of my cotton yarn" to another person), or a more detailed inventory by way of a list or spreadsheet of items that includes the description, location and optional things like the date acquired and the approximate value. If you're already cataloging your stash on Ravelry, GoodReads, some other on-line site for other types of stash, a custom spreadsheet, a notebook, or a legal pad, you can keep right on doing that, and simply refer to it in your memorandum. Generally, it's more important to have a process that works for you than one that might be fancy and all-encompassing, but that you don't actually use.
Next to each item in the list or spreadsheet, indicate both a first and second choice of beneficiary, in case the first one is unable or unwilling to accept the item. This is just a personal recommendation, but if you know a charity that could benefit from your stash, and you aren't entirely sure your named beneficiaries will actually want the stash, I'd recommend listing that charity as an alternative beneficiary. That way, if the original beneficiary gives up knitting or quilting or whatever else the stash is for, and doesn't have a real use for the items and is only taking them to honor your wishes, the beneficiary can decline the bequest, guilt-free, and the items will go to the charity of your choice.
One way to express a charitable preference is something like this:
In the event any of the named beneficiaries is unable or unwilling to accept their items, I instruct my personal representative [or "executor" if your state still uses that term] to distribute any declined cotton or acrylic yarn to Head Huggers, or a comparable charitable organization that provides hand-made hats to chemo patients; any declined baby yarn to Project Linus or a comparable charitable organization that provides hand-made blankets for children in need; and any declined wool yarn to the knitters at my local ABC church, who make blankets for the homeless.
Finally, make sure to sign the memorandum, and if there's more than one page, initial any page that's not signed. If there are any strike-outs or hand-written changes, they should be initialed too.
I strongly recommend putting a date on the memorandum, in case there are multiple versions, so the person administering your estate will know which one is most recent. If you update the memorandum, you can leave the original date, and simply add "updated on _____."
DISCLAIMER: The rules relating to wills and probate vary from state to state, and country to country. The information here is not intended as individual legal advice, but is simply offered as information you can discuss when you consult a qualified professional in your jurisdiction, who can advise you with respect to your local laws and their applicability to your specific circumstances.