Is it one of them one piece fibreglass jobbies ?
Never fitted one to a P38 but have done a classic - I expect it will be much the same
Top tips :-
After you have removed the old headlining wash your hands thoroughly to stop leaving finger marks on the new one (ask me how I know !)
Get someone to help you - it really is MUCH easier with two people, it's not heavy but it is awkward to handle on your own
Lay the back of the back seats forward and recline the front seats as much as you can to give you working room - also helps to slide new lining in "cock eyed" so it clears the tailgate aperture
Apart from that just take your time and you'll be fine
Are you ordering a complete new headliner or are you repairing your existing one. Several repair details can be found on Range Rovers I ordered my replacement headliner material and glue from Auto Carpet - Car Carpet - Automotive Carpet I bought the DIY fabric kit with color 2002-light neutral.