Being in position that required writing of significant number of "how-to" documents, I have had the same need; the solution I found that worked best for me was to use:
MS Office Clip Organizer
It is in the All Programs\Microsoft Office...Office Tools folder in the Start Menu.
using your favorite image editor (or cut & paste from Word to your image editor) to create your custom shapes, images, signatures, etc that you want to stamp your docs with.
a) Save AS: I like to use bitmaps and GIFs ...GIFs are especially nice when you want transparency for things like signatures.
b) Save to a folder in your Pictures folder (i.e.: C:\Users\Name\MyPictures\MSWord_Shapes)
c) Add a prefix your image file names using a common tag, like your initials (i.e.: dw_myshape01.gif)
Open MS Clip Organizer and create a New Collection (i.e.: myShapes) or just use the Favorites collection folder.
a. Then add all of your Custom Shapes to your new collection.
In Word, when you want your custom shapes, use "Insert Clip" instead and just type your chosen prefix (e.g.: "dw" - without the underscore) in the search to pull only your images and insert at will :-)
a. If you want to do more than re-size the image, just un-group and edit the parts. I have un-group and add text box in my quick access tool set for this.
In the end, it is not as quick and easy as having them in the Insert Shapes menu, but it's faster than customizing on the fly.
Using the MS Clips Organizer is not necessarily better than the cut & paste from a shapes template suggested by COT either (a technique I have used as well), this is just another option. And sometimes the way you do things comes down to what best fits your workflow.
Hope this helps!