Need to make sure there is little remaining fuel in the tank, otherwise will be messy and dangerous with fuel emptying out of the sender hole. Best to drive the car until it runs out of fuel and then only put just enough fuel in it to return home to do the job.
Also when you have refitted the sender unit make sure you slowly fill the tank at home using drums of fuel. Sometimes the sender seal does not seat properly and fuel will leak from the tank, especially if you are reusing the old seal. If you are filling using drums of fuel you can stop filling immediately if you detect a leak and then drain the tank again. A while back silly me refitted the sender unit and zoomed straight up to the servo for a full tank. Job done, no worries!....But when I was in the long queue waiting to pay I casually looked around to see fuel pissing out from under my tank. I paid and got out of there quick before the whole suburb got locked down and before knowledgable people started asking lot's of questions. Not a good feeling.