Starting from Signature Cuffs v2.1.1 (and later, Signature Gags) can be rezzed via RLV Shared folders in addition to the built-in Air-Rez. Note that RLV Shared Folders require the RLV to be worn, and there are no cool rez anims.

First things first. Make sure your cuffs are already set-up and you have rezzed at least a full set of "Used" gear via "/1c" or via clicking the relevant HUD button.

Now, on to the little tutorial, courtesy of rommie Vesta (the tutorial's for cuffs, which is more complex - the same ideas apply to the gag; see bottom for more info):

1. If you have never used the RLV Shared Folder feature before, you will need to create a new folder in your inventory.  Open your inventory, click on "Create" at the top of the inventory window, and select "New Folder."  Then rename this folder, "#RLV."  It should look like this in your inventory -> screenshot
2. Now, create another folder and put it inside the first one you made.  Name it something like "Lulu Cuffs".  It should look something like this. -> screenshot
3. For the next step, we are going to do something new.  we're going to make a buch of folders.  ~_^  If you are not wearing your cuffs at this point, put them on now (all of them please), then count how many cuffs you are wearing (you should have either, four, six, or eight).  Then, inside the Lulu Cuffs folder you made, make one new folder for each cuff.  So in my case, I am wearing eight cuffs, so i made eight folders.  So, My inventory now looks like this. -> screenshot
4. Now look in in your  "Recent Items" section of your invetory.  You should see all the cuffs that you are wearing in there.  If you have a lot of recent items, try typing "USED LULU Cuff" in the search field to narrow it down, like so. -> screenshot
5. Now, one by one, drag each cuff that you are wearing into one of the "New Folders" in your "Lulu Cuffs" folder, until each "New Folder" has one cuff in it.  Then stretch your invnetory indow so that you can see the entire name of each cuff becuase it'll make the next step a little easier.  It should look like this.  -> screenshot
6. Ok, almsot done.  For this next part, look at one of the cuffs and look pecfically in it's name were it says, "worn on...whatever".  It should say something like, "worn on l forearm" or "r lower leg"  Whatever it says, rename that folder to the part of the body it's cuffs is worn on.  For example, your right wrist cuff should be worn on your "r forearm"  so the folder for that cuff should be simply called "r forearm".  Once you get each folder done, it should look like this. -> screenshot
   
   (Edit: If you don't care about rezzing each cuff individually, you can have them all rez together under "LULU Cuffs" by naming each cuff folder with the prefix "." - e.g. ".l forearm" or ".r forearm". This tells RLV to hide these folders from view, so that when you choose the "LULU Cuff" folders you will no longer see these child folders - you will instead rez everything in the hidden "." folders).
7. There are a number of items in SL that can access the folder you just made.  To make things simpler, i'll show you how to do it using the Lulu HUD.  First click on the "Lulu" on your HUD and you should get a menu.  Then click on the button that says "RLV..."  (you can also say, "/1.RLV" as a shortcut).  Then, click on the button that says "sh. folders..."  After a moment, you should get a meny that loks like this.  -> screenshot
8. Click on the button with the 1 on it and then you can select either to wear or remove the contents of the Lulu cuffs folder.  it's that easy.  Any of your owners can do this as well!  You can also make other shared fodlers than can also be accessed in this exact same way!  This is great for clothing and even the Lulu Signature gag can be used in this same way too!  Take a look at all the shared folders i have!  -> screenshot

(Lulu: thank you rommie for sharing your lovely butt, er, tutorial, with everyone!)
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If you have more than one type of cuffs (e.g. leather and padded-steel), you can name your main folders accordingly (e.g. "LULU leather cuffs" etc.), and put the relevent cuffs in those folders.

If you want to rez cuffs in pairs or groups, you can create new main folders accordingly (e.g. "LULU wrists", "LULU ankles") and put relevant cuffs in those (but add "." in front of the child folders e.g. ".l forearm" and ".r forearm" so that RLV will rez everything in those hidden "." folders all at once when you choose "LULU wrists", instead of giving you the choice to rez "l forearm" or "r forearm" separately).

If you have multple gags, you can name your main folders according to your gag names (e.g. "LULU ball:strap") and remember to create a subfolder called "mouth" under each of the main folders.