As Wendy says you lean on the crutch and let your arms take some of the weight your legs might not be able to support. You just walk normally – not swinging through because you don’t want to take weight off your feet just add an extra support if you wobble.
My physio told me that the most dangerous time with crutches is sitting down and getting up. You have to remember not to have the elbow support on at this time because you could break your arm if you loose your balance. (I found it hard to remember to take the crutches off before sitting down but I can see how this could cause injury)
I can’t use 2 crutches because I have poor control over my left arm and a single crutch causes me to lean to the right and eventually causes neck pain but I think they add more firmness if you feel unsteady. I also found people were more ready to help or move out of the way when they saw the crutch. It’s a bit like the scooter/wheelchair thing – a crutch is more serious than a stick.