If you have a word, that ends in the l sound, and it is one syllable, then you need to put two ll
full, all, bell, tall, till, ill, still
But if you add another syllable to the word, and the l sound is at the end, then you only put one l
careful, cheerful, until
If you have a word that usually has two ll like all and well and you add another syllable after it, then one l is dropped
already, also, although, always, welfare, welcome,
Some words with one l get another when an additional syllable is added:
cancelled, cancelling, signalling, metallic.
But there is always an exception in English: signaled has only one l
One word that has a difference between American usage and British usage is
traveller, is the UK spelling with two ll and traveler is the USA spelling.
Lesson Plan by Rachael Alice Orbach - Professional English teacher