Fairly simple task; first step, lock / hold the steering wheel in a central position.
Then, remove the large nut from the pitman arm.
Next, mark the pitman arm position on the splined shaft for the reinstallation in the same position.
Then, using a pitman arm puller - remove the pitman arm from the steering gearbox splined shaft.
Next, place an oil drain pan underneath the steering box to capture fluid. Then, using a self-tapping screw - penetrate the existing seal and pull out the seal from around the gearbox splined shaft - using a pair of pliers.
Place a film of transmission fluid on the new seal for lubrication and install the new seal (being careful of the direction) over the shaft. Tap until seated using a suitable size socket - maybe deep socket sized to just fit inside the steering gearbox diameter.
Reinstall the pitman arm on the spline in the correct position using the index mark. Install the pitman arm nut and torque to specifications.
Try the many on-line auto parts sources; such as - www.autopartswarehouse.com, www.partsgeek.com or www.rockauto.com for the replacement parts.