Rubber seal leak

[Curlytoppz]

Got a little leak on the drivers side.  Think it is from the window seal as there is a little water there and it is leaking onto the drivers seat.  I had this problem before on the other side and the garage just adjusted the seal as it was hanging too low.  It all looks tight but looks like the seal is starting to perish.

My questions are:  Are these really easy to fit (ie a dumb ass like me can slot them on) or a big job (ie taking them to the garage to be fitted) and how much would I expect to have to pay for new ones?

Thank you in advance   s:D s:D