You'll need a valve spring compressor, and you'll need to remove the head, the seal is a simple cap, but you'll need to remove the spring to get at it. Use the spring compressor to compress the spring, then remove the spring retainers from the valve, then slowly release the spring compressor and take the spring out. The seal is sitting on the bottom of the valve, just slide it off and put a new one on and do everything in reverse to put it back together. Its not hard, only hard part is getting those pesky retainers back in right.
Heres some pictures to help you out, not a ranger but close enough