Button / function guide for Amazon Firestick remote with DirectTVstream

There is no guide. It's trial and error, like with most devices and streaming services. Experiment with the up/down/right/left of the circle when watching, etc.

It seems you would be better off with a DIRECTV STREAM box if you wish to have a physical Guide button as well as identifying channels by their respective numbers. 
A Fire Stick ( as well as other streaming devices) does NOT have these features, and your mother will have better luck with a traditional cable/satellite feel (which the DIRECTV STREAM box does hands down offer).

There used to be guides for the other devices - not great but at least something.  For some reason these were removed some years ago.  (I've tried to find them on the Wayback Machine, but so far unsuccessfully.)

For most of the devices, the basic functionality isn't too complex:

• When viewing a video, pressing "select" (generally the center of the d-pad) brings up an overlay with info and icons (for recording, setting closed captions, etc.) that should be fairly self-explanatory
• Also when viewing a video, pressing up (or down - it seems to depend on which type of device) brings up the "home" screen, which provides access to the program guide, the DVR recordings, the on-demand content, search, as well as direct access to a variety of content (by selecting "tiles".  Again, the items should be fairly self-explanatory, but experimenting will likely be needed.
• Also from a video screen when watching "live" content (a channel), the left and right buttons normally "surf" from channel to channel.  When watching a recording or on-demand, I'm not sure if those buttons behavior is that consistent.
• The pause, rewind, and fast forward buttons do what they should, within the limits of what is allowed for the various content types.

If you have specific questions, I'm sure someone here can help (aside from the question of why they removed the earlier help pages...)

BTW - the "DirecTV stream remote" is specific to their streaming device.  You can't buy one separately (except on eBay) and it wouldn't help you with a Firestick.