This is what I usually explain in class: the comparative/superlative form depends on the length (
la longitud) of the adjective or the number of syllables; I hope you'll see this chart as useful as I tried to do it for you.
Comparing qualities
This is what you do when you compare 2 things or people; you use an adjective and you should have the comparative form according to the number of syllables of the adjective, but the general rule is that you shouldn't have more than 2 syllables after adding the particle. Keep in mind that the adverbs are a bit different when they end in -LY. As a reminder (recordatorio)  an adjective is a characteristic of something or someone (with nouns) and an adverb is a characteristic of an action (with verbs)
Look at this chart: 
| 
1 syllable | 
2 syllables | 
3 or more syllables | 
| 
You add -ER THAN 
 
clean = cleaner than 
big = bigger than yours 
dry = drier than 
 | 
Ending in -y = -IER THAN 
 
easy = easier 
friendly = friendlier 
lovely = lovelier | 
You use MORE --- THAN 
 
expensive = more expensive than 
beautiful = more beautiful than 
 | 
| 
Exceptions: 
more bored than (boreder???) | 
Ending in consonant =  
MORE --- THAN 
careful = more careful than 
boring = more boring than | 
Adverbs ending in -LY = 
MORE --- THAN 
easily = more easily than | 
 
The superlative form is very similar:
 
| 
1 syllable | 
2 syllables | 
3 or more syllables | 
| 
You add  THE  -EST 
 
clean = the cleanest 
big = the biggest 
dry = the driest 
 | 
Ending in -y = THE  -IEST  
 
easy = the easiest 
friendly = the friendliest 
lovely = the loveliest | 
You use THE MOST --- in ... 
 
the most expensive car in the market 
the most beautiful picture in the museum 
 | 
| 
Exceptions: 
the most bored child  | 
Ending in consonant =  
THE MOST --- in the ... 
careful = the most careful person 
boring = the most boring film | 
Adverbs ending in -LY = 
THE MOST --- in ... 
easily = the most easily built | 
Apart from these, remember there are
 irregular comparatives and superlatives:
 
 
 
 
  | ADJECTIVE/ADVERB | COMPARATIVE | SUPERLATIVE | 
  | good / well | better  (than) | the best | 
  | bad / badly | worse | the worst | 
  | far | farther / further | the farthest / furthest | 
  | little (for quantities/qualities) | less | the least | 
  | much / many (plural) (for quantities/qualities)
 | more | the most | 
If you want to practise I really recommend doing comparisons orally. You can start with:
- a dog, a cat and a canary
- a Mercedes, a Fiat and a Ferrari
- a car, a bicycle and a bus
- a house, a flat and a caravan
Example: I prefer a dog because it's nicer than a cat but a cat is more independant. A canary is the dirtiest pet but it's the easiest to have in a flat. A dog needs more room and more care than a cat.
If you need more practice with exercises, you can do it if you visit these webpages:
 
To finish with, there is a comparison of equality and it's formed with  AS + adjective/adverb + AS like in the following examples:
- My brother is as tall as my father
- A caravan may be as confortable as a small flat
Hope it's clear and you can use it!