What should i know when replacing a head gasket?

What should i know when replacing a head gasket on a 2005 Ford E250 4.6L V8, seems like an easy job. Was wondering if i should know any special i should do when taking off and putting it back on

any help is appreciated

You need to skim the head if you replacing the head gasket, find someone near you that can do it should cost around £30-£50

Just replacing the gasket more likely will not fix it if it got hot the heads will have to be resurfaced and that takes a little bit of work if you don't reseruface them it will never seal this I a big job