The extension housing oil seal unlike other parts can be done without the transmission being removed from the car. It is advisable to start the operation by lifting the car up to a comfortable working level using the jack and supporting it by means of readily available jack-stands. Empty the transmission of lubricant as required. Next, remove the driveshaft. Using seal removal tool or simple large screwdriver remove the old worn out seal and clean the counter bore to assess the extent of the damage. On the outer diameter, you have to put some sealant and in the region between two lips of the new seal, there should be an application of transmission oil. It is best to push the seal on the flange with the lip turned in until the flange grabs the seal, it can be forced with a seal driver, a large pipe wrench or a socket wrench or with a section of pipe of the proper outside diameter. Following installation ensure that the driveshaft is reinstalled, then you make sure that you fill the transmission with the appropriate lubricant.