Turn the water off to the faucet.
There will be a screw that holds the handles in place. This screw may be under a plastic cover that pops in place.
You will most likely need a faucet handle puller to remove old handles.
Once handle is off, You need a valve stem socket set to match the size of the stem you need to remove. Remove the stem.
Now there is a little seat in the valve. It is either hex or square. You need the right sized seat extractor tool to remove it.
Take the seat and stem to a large hardware store and try to find the exact replacement.
Install in reverse order.

If you have none of these tools, you'll spend under $50 for them.

Have fun...