There are only a few words which you should pluralize by changing the f or fe to v and adding es. They are as follows:
Singular ending in f
- calf to calves
- elf to elves
- half to halves
- hoof to hooves
- leaf to leaves
- loaf to loaves
- self to selves
- sheaf to sheaves
- shelf to shelves
- thief to thieves
- wolf to wolves
- yourself to yourselves
Singular ending in fe
- knife to knives
- wife to wives
SpanishDict.com, from which I obtained most of this list, also lists ourself to ourselves and themself to themselves, but since them and our are plural, I don't believe ourself and themself are legitimate words.
So, keep this list. You could probably even memorize them. By my count, that's only 14 words.
You must also remember that if a compound word ends with one of the words that uses the ves ending to pluralize, the compound word will do the same. For example, knife changes to knives, so penknife will change to penknives, and wolf changes to wolves, so werewolf will change to werewolves. There is a list at en.wictionary.com which includes quite a few of these kinds of compounds.
To pluralize the other words ending in f or fe, simply add s.