u'll have to pull out the door panel, then take out the window. and then pull out the weather strip.

before putting the new one on, clean the parts taht it slides on to (just wipe em off w/ a towel. Then very lightly spray some "WD-40" on those parts so that teh strips would slide on very easy.
Use WD40 because it evaporates, most other lubricants dont.

Then after u're done putting the strip on, lubricate the inside of the seals. This will help the windwo slide up and down easy. Then put everything back together.