Use the dial on the front of the slow cooker, @jakedduck1. Usually it will take four hours on high (if the roast is not too big). The carrots, onions should be scraped and cut into quarters, the potatoes should be cut in quarters (unless they are the small potatoes), then just cut them in half. The veggies go on the bottom of the slow cooker. I usually brown the roast in a skillet before placing in the slow cooker. That's my best advice. It should be wonderful! You will feel proud of yourself when it comes out of the cooker and it will smell wonderful. Next time you make it, you can invite company over.😉
PS The cooking bags are great, I use them all the time. No clean-up needed.