If they're not commonly used, write them in italics:
Just use “e” or “ae” spellings, not "æ":
Use plural forms if they're commonly used:
But avoid using "alumnus" and "alumni" wherever possible - see Alumni, alumna, alumnus to find out why.
And be wary of words with more than one plural form with different meanings:
"They may have found the formulas for a long and happy life, but we're still looking for the maths and chemistry formulae."
Avoid Latin abbreviations like "eg", "etc" and "ie". See Abbreviations and acronyms for better alternatives.