The dryer will need to be unplugged and you will need to remove the drum to replace the rear felt seal. The first two images below show how to access and remove the drum in a common type of Kenmore dryer. The third image shows how to remove the drum in a different type of Kenmore dryer. There are several different types of Kenmore dryers. If you have a different type and need help removing the drum, resubmit your question with additional details and the full model number of your Kenmore dryer.
Once you get the drum out, you will need to remove the old seal. You likely received high temperature glue with your new felt seal. This is not an easy repair. The new felt seal will need to be carefully positioned and sometimes properly stretched to fit the drum. I normally recommend folding the new felt seal in half to find the middle point. Carefully glue this middle point onto the drum. Work the two sides of the felt seal equally around the drum to complete the installation. Make sure that the glue dries fully before reinstalling the drum.
If you need more help, resubmit your question with additional information and the model number as noted above.