Hi, For the extra cost involved I'd have them installed professionally. Granite worktops are a luxury item and people usually expect them to be fitted well. They are also darn heavy and can be surprisingly fragile until fitted.
You can fit them them yourself if you are determined, but cutting them isn't a DIY job. You need specialised kit with diamond cutters to get it right. Whatever you do don't believe the stories about old Fred down the road with his ancient angle grinder who cuts it by eye with total accuracy every time!
If you need anything other than a dead standard shape, make a template the exact size and shape you want from ply and take it to a stonemason who makes worktops and get them to cut and deliver â€“ trust me, you don't want to put it on your roof-rack. Don't forget the overhang for the cabinet doors/drawers.
Make sure the sub-base (what you are laying the granite on) is good and strong. if this is for a kitchen or bathroom use good quality base units bolted together with a sub-base of ply (or MDF). You may need to modify the units a bit. Just sticking the sub-base over top may result in a gap between the top of the doors/drawers and the worktop.
You'd think granite would be dead flat. Well it ain't, not quite. And even if it is the sub-base won't be. So you will probably need to shim the granite a little bit to get the level right. Use proper shims, not a couple bits of folded fag packet. It's unlikely to go anywhere, but when finally laying, a few dabs of epoxy twixt top and sub-base will ensure it doesn't. Where the granite meets the wall, or where two slabs of granite meet, use matching coloured epoxy.
As I said, if you can afford granite in the first place you can afford to to let someone else have the pain of fitting it!