how do i change the sump gasket on my 1994 2.5 diesel ford transit?

Can't add much more detail really. Just bought the van (with the leak and replacement gasket). It's a long wheel base custom D 190 hi top. Any help would be much appreciated as I'm a have a go type, but would like to know if it's going to be a real pain cos i've been told it'll cost at least £70 - £100, and i need to weigh up the pros and cons.