If it is not a Fantastic Fan then it will be a standard vent,It is pretty easy to do your self, first you need to know the type you have then get what is needed.
Here are the types of vent lids,by the hinge type.

check yours then you will know which one to get.
I would suggest getting the
Polycarbonate ones they last a lot longer. Also something like the
Maxxair covers,they will make your covers last longer and you can keep your vents open in any weather.
If you haven't done it before
Here is a short video on how it is done.