I woɾk in ɑn indiɡenous lɑnɡuɑɡe in S. America. Much of what I need is the same as for Spanish but I ALSO need to be able to key a few special characters (latin letters with diacritics) not used in Spanish. My computer is a Dell Latitude D630 running Mint 17 "Qiana". Its physical keyboard is English (US).
Can someone tell me where to find instructions for modifying my keyboard to meet my particular needs?

There are two approaches.

1. Use the AltGr (Level 3 Shift). You can then add a third level to suitable entries in the file
/usr/share/X11/xkb/symbols/us
For example, my file has
key <AC01> {[ a, A, schwa, SCHWA ]};
so that AltGr+a gives ə

2. Use the Compose key. Your keyboard configuration tool enables you to set a compose key: the Windows menu key is a convenient choice. Then you can use the compose sequences: see Wikipedia. Thus
Comp ~ a gives ã
Comp ; a gives ą

The compose sequences can be customised. Firstly you have to use the input configuration tool to set the input method to xim. Then you can have a custom file $HOME/.XCompose with lines like
<Multi_key> <c> <j> : "ǰ" U01F0 # Latin small J with caron
so that "Comp c j" gives ǰ

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Hi and welcome to LQ.

1. Be sure to use the UTF-8 locales. That will give you all the special characters.
2. This link shows how to configure the keyboards on Debian (should work on Mint as well).

What a coincidence! I also worked on an indigenous language in Colombia in the comisara del Vaups along the Papury river. The language was Desano (a Tucanoan language) and I was there in the early 1960's. It was a wonderful experience and the people were really great. They even adopted me.
I hope you're having lots of fun but don't drink too much chicha.

jdk

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Also, if you need characters not in your default fonts, these are useful places to look for them:
http://www.wazu.jp/
http://www.unifont.org/fontguide/

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Thanks, both of you who responded! That put me on the right track. A friend had sent me a custom Compose file a while back but for some reason the file name got changed so it wouldn't work. When I finally realized that was the problem & corrected it I was able to access it, then tweak it a bit for our particular needs. I think we're up & running now, with just one irritating limitation.
I'd still like to learn how to customize the "US" keyboard layout's variant called "US, international with dead keys," as it looks like it might be even better than what I now have -- if I could reassign some of the key combinations. For instance, I need the apostrophe & double quote mark w/o having to key them with the Rt-Alt key.
Thanks again for helping an older "newbie" (the hardest ones, I imagine! "hard to teach an old dog new tricks").

You can reconfigure your keyboard as needed. For debian based distros:

$ sudo dpkg-reconfigure keyboard-configuration

And be sure to have the fonts and unicode/locales stuff installed for the non-utf8 type languages.

The apostrophe/quote problem is one of the reasons for not using the "dead key" version. Have a look at the version without dead keys, and customise it. The file is
/usr/share/X11/xkb/symbols/us
It's easy to alter, but always keep a backup — one missing ";" and you've got no driver! For example, I have used the third and fourth levels (AltGr, AltGr+Sh) for phonetic symbols on my gb layout:
key <AD03> {[ e, E, U025B, U0190 ]}; // Ɛ
key <AC01> {[ a, A, schwa, SCHWA ]};

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