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
- beef to beeves (or beefs)
- calf to calves
- corf to corves
- dwarf to dwarves (but only in Tolkein)
- elf to elves
- half to halves
- hoof to hooves
- leaf to leaves
- loaf to loaves
- ourself to ourselves
- scarf to scarves (or scarfs)
- self to selves
- sheaf to sheaves
- staff to staves or (staffs)
- shelf to shelves
- themself to themselves
- thief to thieves
- wharf to wharves (or wharfs)
- wolf to wolves
- yourself to yourselves
Singular ending in fe
- knife to knives
- life to lives
- wife to wives
Also, Oaf to oaves appears to be an archaic form, with oafs being correct now. Ditto roof to rooves, with roofs being the current accepted plural form.
So, keep this list. You could probably even memorize them. By my count, that's only 21 words, and a few of those are generally uncommon.
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.