Loosen the tensioner and carefully slide the old timing belt off the pulleys. Take note of the belt’s routing as you will need to replicate this with the new belt.
The temperature within the oil seal environment is the main factor that dictates the composition of an oil seal. Generally, there is a range of set temperatures that are optimal for that material. If the operating environment is too cold, the seal may become brittle and, at high temperatures, the seal material may show increased elasticity and become leaky.
Fitting an oil seal

Please see the following for the types of sealing devices for bearings.
How to Select the Right Bearing (Part 7): Components surrounding the bearing
Runout must be kept to a minimum. Movement of the center of rotation is usually caused by bearing wobble or shaft whip. When coupled with misalignment, this problem is compounded. Contrary to popular belief and common practice, the installation of flexible couplings cannot correct or compensate for misalignment.