My mum did a few floral art classes in the 70's, so I was a bit apprehensive about her abilities. I was so pleased with my flowers though. I'm sure the HIB's will have better advise, but this is just what we did.
We did a trial run a week or so beforehand & kept the bouquests/buttonholes in a few locations: 1 in the fridge, 1 in the cellar etc & compared which kept best. Its also a good idea to see which flowers/foliage wilts the quickest, & therefore may not be the best if you are making the bouquet the day before the wedding. Orchids & roses last a long time in bouquets.
Hobbycraft sell several floristy bits including wires, green tape, oasis for table centres and oasis on a handle for bouquets. There is also a spray you can use on flowers & leaves which prevent them loosing moisture & stay looking fresh.
There are many online classes & pics to help. My mum did the majority of the work, but myself & a friend helped wire everything, cut flowers etc. I onyl had 1 bridesmaide so we made: a trailing bouquet for me, a hand held for my BM, a throwing bouquet made with excess flowers & 4 buttonholes. We bought the flowers on Thur: soaked the oasis overnight & sat the flower stems in large buckets in the cellar, On the friday we took the oasis out of the water to get rid of the excess, wired all the leaves & flowers & assembled the bouquets & had the wedding on saturday.
Because the buttonholes weren't sitting in oasis, they wilted 1st, so mum redid them on the day.