I thought about doing DIY flowers, as we are doing DIY for a lot of things, but decided against it, as I can't even make a bunch of flowers from the supermarket look nice!
However, if you have a look on Pintrest, there are lots of tips for how to do your own bouquet, and I think there is an article here on Hitched too.
If you're looking to save money, I would recommend scouting the supermarkets - some of them do beautiful bouquets for around £10-20 - you could buy them a day or so before, remove the cellophane, wrap the stems in lace or ribbon, and stick them in water overnight? Hmmm... I may have just talked myself into DIY flowers actually!
My friend used supermarket flowers for her centrepieces too - she just bought them the day before and stuck them in jam jars - it looked lovely! We're doing something similar, but future MIL is growing the flowers on her allotment.
Hope this helps somewhat! X