Is it hard to fix an iphone yourself?

I'm buying a glass replacement on amazon for a cracked screen. Would I be able to do it with out messing it up if I watch a how to youtube video or should I get someone to do it?

It's a straightforward repair if you take your time and pay attention to a good tutorial.

Note that you can only separate the glass and LCD on the iPhone 3G and iPhone 3GS models. On the later models, you need to replace the full screen assembly which includes the glass, digitizer and LCD screen. Apple does not sell their parts in the market and I know from experience that not all replacement screens are made equal.

After sorting and filtering over the years the best company I recommend is:
http://www.GadgetMenders.com

They carry the highest quality replacement parts and tools and provide great video tutorials.