New to couponing!

I'm trying out couponing and have a few questions! Please don't laugh if they sound obvious but I can't find the answer anywhere!
What's the best way to use them?

I've signed up to Super Savvy me and printed off my first lot. My first thought was "buy everything at Wilko as it's generally cheaper so I'm getting more out of my coupon." But do Wilko accept them? And are they generally cheaper?
Or do I just get them from the supermarket? Can I use them when the item is on special offer even though it says on the voucher that you can't use it in conjunction with any other offer? Because I've read that this is just a myth!

For example, I have a voucher for shampoo £1.50 off. If it's on offer for £1.50 and I handed my coupon over, would I get it for free? Or does it have to be full price for the coupon to work?
Any other tips gratefully received!