Firstly, כ is not a guttural letter, but simply a soft letter. "Guttural" means pronounced in the throat.
In modern Hebrew, they are pronounced the same, like a harsh scratching noise at the back of the mouth.
In the original pronunciation (preserved by many Jews today), כ is pronounced in the same area of the mouth as hard kaf, with air hissed through a narrow opening. ח is pronounced by squeezing the throat muscles and exhaling, which is difficult for many English speakers to replicate.
When you read Hebrew texts often, you begin to develop a sense for when כ is used and ח is used. To help you begin to develop this sense, כ is used very often in prefixes and suffixes, while ח is only used as part of a shoresh (root word).