First we need to understand what vowel and consonant suffixes are.
vowel suffixes: -ance, -er, -ing, -able, -ible, -est, -ed, -ation
consonant suffixes: -ly, -less, -y, -ship, -ment, -ness, -ful, -s 

Summary of some key rules ( but like all spelling rules there are always exceptions)

1.  -y to -i- rule with words ending in consonant + y.

a. Change the y to i before vowel suffixes (except -ing) 

carry + ed =  carried 
carry + er = carrier
carry + age = carriage
BUT carry + ing = carrying (can't have two i's carriing!!!!! But exceptions see rule 5)

happy + er = happier
happy + est = happiest

supply + ed = supplied
supply + er = supplier
supply + es = supplies
BUT supplying

deny - denied, denial BUT denying
marry - married, marries BUT marrying
rely - reliable, reliant, relied, relies BUT relying
comply - compliance, compliant, complied, complies, BUT complying

b. Change the -y to -i- before consonant suffixes

beauty + ful = beautiful
beauty + fy = beautify

happy + ness = happiness
happy + ly = happily

angry → angrily, angriness
ordinary → ordinariness, ordinarily
lonely → loneliness
empty → emptiness
plenty → plentiful
likely → likeliness, likelihood
necessary → necessarily
busy → business, busily
hungry → hungrily

c. exceptions - dryer, drying, dryish, dryness, slyness

dry + ly = dryly or drily

2. Drop the 'e' rule before a vowel suffix

a. Words ending in silent 'e'  + vowel suffix

love → loving, lover, loved, lovable (or loveable)
make → making, maker
use → using, usable, user, used
have → having
adore → adorable
care → cared, carer, caring
move → movable, moving, mover

b.  Exceptions: Words ending in -ce or -ge keep the 'e' before -ous and -able ( because it softens the g “juh” and c ”s” sounds).

courage → courageous
service → serviceable
changeable
gorgeous

manage → manageable  (BUT drop the 'e' with -ing, -ed, -er:  managing, managed, manager)

notice → noticeable  (BUT drop the 'e' with -ing, etc. noticing, noticed)

Keep the 'e' rules:

a. Keep the 'e' with consonant suffixes

Keep the 'e' before -ful

care+ ful = careful
hopeful, peaceful, useful, wasteful

BUT change words ending in consonant + Y to i

plenty + ful = plentiful, beauty + ful - beautiful

b. Keep the 'e' before -ly (consonant suffix)

accurate+ ly = accurately, like + ly = likely, love+ ly = lovely, definite + ly = definitely

lively, rarely, homely, desperately, approximately, entirely, completely

BUT due - duly, subtle = subtly, true - truly

c. Keep the 'e' before -ment  (consonant suffix)

achieve + ment = achievement, advertisement, arrangement, excitement, involvement, replacement

BUT drop the 'e' with argue + ment = argument.

Also acknowledgement or acknowledgment can be spelt either way.

judgement usually keeps the 'e' but lawyers spell it without the 'e' judgment

d. Exception: Keep the 'e' when the word ends in -ee or -ye 

agree → agreeing
eye → eyeing
see → seeing
guaranteeing, foreseeing, decreeing

3. 1:1:1 rule - Check out the 1:1:1 doubling up video

a. Words of 1 syllable ending in 1 vowel+ 1 consonant - double the last letter before a vowel suffix:

shop → shopped, shopping, shopper
plan → planned, planning, planner
fat → fatter, fatten, fattest, fatty
pot → potting, potted, potter, pottery
slip → slipped, slipping, slipper, slippery

Do not double last letter if it's a w, x, y: tax → taxed, stay → staying; towed
Do not double if two vowels: heat → heating; sleeping, sleeper
Do not double if two consonants:  sharp → sharpen; fasting, turned

BUT qu- is classed as one consonant because we can't have q without the u
quit → quitting, quitter, quitted
quiz → quizzed, quizzing, quizzer

b. Words of more than 1 syllable ending in 1 vowel + 1 consonant when the stress is on the last syllable double up the last letter.

regret → regrettable, regretting, regretted
forget → forgettable, forgetting (past: forgot - forgotten)
occur → occurring, occurrence, occurred
begin → beginning, beginner
commit → committing, committee, committed
forbid → forbidden, forbidding

prefer → preferring, preferred (BUT preference, preferable, preferential- the stress moves)
refer → referred, referring (BUT reference, referee- the stress moves)
confer → conferred, conferring (BUT conference)

(No change if stress not on last syllable - (stress on first syllable) target → targeted, limit → limited, limiting, budgeting, marketing, marketed)

BUT focus can be both focusing or focussing, focused or focussed

ALSO  input → inputted, inputting
kidnap → kidnapped, kidnapping, kidnapper (American English can also have kidnaped, kidnaping, kidnaper but these are less common now.)
output → outputted, outputting
worship → worshipping, worshipper, worshipped ( American English can also have worshiped, worshiper, worshiped but these are less common now.)
handicap → handicapped, handicapping, handicapper

4. Double the last L
Words of more than 1 syllable ending in 1 vowel + L always double up the L before a vowel suffix (except after -ity and -ise). American spelling mostly  don't double up. 

travel →travelled, traveller, travelling) (American → traveled, traveler, traveling)
cancel → cancelling, cancelled, cancellation (American → canceling, canceled, cancelation)
label → labelling, labelled (American → labeled, labeling)
model → modelled, modelling, modeller (American → modeled, modeling, modeler)
marvel → marvellous, marvelled, marvelling (American →marvelous, marveling, marveled)
signal → signalling, signalled, signaller signally (American → signaled, signaler, signaling)
quarrel → quarrelling, quarrelled, quarreller (American → quarreling, quarreled, quarreler)

enrol → enrolling, enrolled (enrols) (American enroll - enrolling, enrolled, enrolls)

expel → expelled, expelling, expeller, expellable (American the same)

BUT: devil → devilish, loyal → loyalist, parallel → paralleled, paralleling, travel → travelogue

Do not double when two vowels: appeal → appealing, appealed

5. Words ending in vowels  - no change with vowel suffixes.

echo → echoed, echoer, echoing
henna → hennaed
mascara → mascaraed
radio → radioed
ski → skied, skier, skiing
taxi → taxied, taxiing
video → videoed, videoing
These words are from other languages or new words like video, radio.

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