They feel like your whole tummy is tightening then releasing but associated with pain.
Only the thing I can say is... you will know! For me, I had back contractions wi the my first one and it felt like the worst cramps from a period but in my lower back. I could only curl up and cry. When they moved to my stomach, they felt like I had worked out far too hard and my muscle were in a constant state of pain. I think its different for everyone, but you will know when its time.
Thats really the only way to describe it.. It'll feel like you have to use the bathroom and poop also.. for me it did at least. It's hard to stay in one position but it hurts either way.. standing .. sitting.. laying down.. walking.. at first they are manageable but afterwards when they become stronger and closer together you will want to cry. How often are you having them?