-l- - -ll-

Problems exist for many writers of English in the spelling of double consonants at the ends of words, particularly in the inflections of verbs. This is particularly marked in the case of verbs ending in '-l'. Pocket Fowler's Modern English Usage (2 ed.) (on line) explains s.v. -l-, -ll-:

"Much confusion is caused by differing spelling practice in Br[itish] E[nglish] and Am[erican] E[nglish] in verbs of two syllables pronounced with the stress on the second syllable, e.g. enthrall/enthral and fulfil/fulfill. Practice varies even within each variety of English." There follow two lists which form the starting point for AWE's category of single or double '-l-' and category:-l- or -ll- before -ment?.

- to which the fuller printed version (Burchfield's Fowler) adds "The 20[^th] c[entury] has witnessed much shuffling and reshuffling of -l and -ll in the simple form of a good many verbs. ... Truth to tell, no firm rule can be applied" (AWE's emphasis).Burchfield suggests that "a fairly safe guide is to use annul, appal, befall, distil, enrol, enthral, extol, fulfil, install and instil, in Br[itish] E[nglish], and annul, appall, befall, distill, enroll, enthrall, extol, fulfill, install and instill, in Am[erican] E[nglish]"; but this is no rule, merely lists of words that should be treated; and it is one that is changing over time; and Burchfield himself says "all of these words exc[ept] annul and befall may turn up with either -l or -ll in good sources in either country."

• • Burchfield adds further refinements.
    • He notes that '-l' is treated differently from other consonants in final position (see Consonant doubling for more). With final '-l', the position of the stress is immaterial: in British English, '-l' is always doubled before inflections; except that before -ish, -ism and -ist, it is left single (e.g. devilish, liberalism and naturist). In American English, final '-l' is usually left single, except where both varieties leave it doubled before -able (e.g. controllable).
    • "Before -ment, the usual spellings are annulment, enrolment, enthralment, extolment, fulfilment, instalment, and instilment in Br[itish] E[nglish]; and annulment, enrollment, enthrallment, extolment, fulfillment, installment, and instillment in Am[erican ]E[nglish]."
    • The guidance above applies where the single or double final '-l' follows a double vowel (e.g. failed, peeling and boiler), or a consonant (e.g. curled and howling). "Another exception is paralleled, -ing", to which [un]paralleled should be added.
    • Note that compounds and derivatives of words ending in double '-l' sometimes (but not always) drop one of the '-l-'s: almighty, almost, already, altogether and always. (AWE recommends you NEVER to use alright in academic writing.) Skilful and Wilful are the British spellings; in America it is usually skillful, and may be willful. Chilblain has a single '-l-' in both varieties. OED, and Fowler (all editions), prefer the double '-l-' for dullness and fullness, by analogy with illness, smallness and stillness, etc; though in 1897 OED observed that dulness and fulness have "hitherto been more prevalent". They are still common in American English.

AWE's advice is 
1) look up the House Rules under which you are writing;
2) follow the practice of your reader (teacher or marker, if you are a student). 

   Otherwise, take Burchfield's suggestions for British spelling above - and good luck!

You may also want to see -ful and/or -ly.