I had a stab at this a few weeks ago, needless to say I removed it as it hadn't tacked properly to the very bottom of the screen (the bit that disappears into the front clam and was down to me not degreasing it properly) but I did find it hard to get it flush at both top corners and along the top window frame where it drops in between the rubber seal as it kept creasing up in there.
is there any advise to getting these bits nice and tidy, it didn't put me off trying again so will be having another blast now i had a better idea of what i'm doing
The best advice I can offer is use heat. A hairdryer will allow you to heat it enough to stretch it. Be careful not to over strtech as it will rip. They say the more expensive the vinyl the more stretchy it is but I used cheap stuff and it worked well.
I did my windscreen and it was okay ish but I also did my dash plate and the panel betwen the door and air intake vents. Oh and my petrol cap surround. They are as good now as they were about a year ago when I did them.
I then changed my rear view mirror and cracked the windscreen in the process. This meant a new exige black surround windscreen (retails at nearly £1500) for £75 insurance excess. 
Keep trying; it gets easier.