Whenever I get talking to a teacher about spelling, I offer them my brilliant idea that every school should, in the first couple of weeks of the first term, have a "-tion Day" when each first year student and teacher will adopt a useful word ending in -tion. By the end of the day they will know what it means, how to say it naturally, say it for spelling, and, of course, spell it accurately.
I always end that conversation by saying something like, "I know there are other ways to spell the "shun" (or "zhun") ending, but <tion> is by far the most common." I've been making that assertion based on experience rather than research so thought I'd dig a little deeper to back up my claims.
If you like finding word patterns, then you probably know morewords.com or thefreedictionary.com 
I usually get to them by searching for something like "words ending in" or "words starting". This time I got a huge list and scooped off just over 700 I thought would be the most common, put them in alphabetical order and sorted them by <tion>, <cian>, <ssion>, <cion> and <sion>.
As always, memory is required for spelling but we can also play the odds a little.
<tion>
According to my list, 83% of the time, when you hear /shun/ or /zhun/, the spelling is <tion>. On that basis, you don't really have to do much sorting and looking for patterns unless you really want to and your students find it helpful.
- if you hear /ayshun/ then write <ation>.
- roots that end <t>: attract/attraction etc but that doesn't account for all of them because of abolish/abolition.
- absorb/absorption - subscribe/subscription - describe/description - all reflect a shift in pronunciation from /b/ to /p/. In a simple spelling lesson it's better to just think of them as the 'norm' with a <tion> spelling. With your more confident spellers, you can go deeper through word study or by looking at phonetics and pronunciation. For unconfident spellers, that kind of study is likely to be a barrier to learning.
Instead of trying to sort the 83% spelt <tion>, let's look at the remaining 17% - around 120 words with their various spelling options. I've left some words with "no obvious (to me) pattern". If you can find (or already know) a useful pattern for those words, please share in a comment. A pattern is different from a rule or an etymological fact.
<cian>
is about jobs music/musician, optic/optician - even more fun if you're working with someone called Ian.
<ssion>
largely have roots that end <ss> impress/impression and a few that end <mit> admit/admission.
<cion>
There are only two common words with this spelling: suspicion and coercion. So learn those two words and exclude <cion> from the list of options.
<sion>
This is where you might have to use more memory but there are some good patterns.
- listening for /zhun/ rather than /shun/ helps.
- if the verb is spelt with a split digraph plus <d> then it's <sion>. Lots are accounted for with this pattern. collide/collision
- also if it ends in <d> by itself. These ones sound like /shun/. apprehend/apprehension
- roots ending <vert> invert/inversion. It might be easier just to hang all those words on one familiar one: version.
- likewise, any word ending <vision>: revison, television
- a handful of roots ending with /r/ or /l/ with no vowel added. adhere/adhesion (as opposed to adore/adoration) compel/compulsion
- roots ending with the sounds /s/ or /z/ when no vowel is added. precise/precision, transfuse/transfusion.
- "No vowel added" means we have confuse/confusion (no vowel added) but accuse/accusation (vowel added).
Important!
Please don't teach these as rules. Instead, use the information to have a look at the 62 words that end <sion> and ask your students what will help them remember. Is it listening for /zhun/? Is it thinking of the root? Is it both? Is it something else?
Trying to recall rules isn't a great use of memory. Instead, try to see and remember patterns.
Here's the pdf: tion-sion-ssion-cian-etc-word-list