I have four kids under 7 and have been to WDW 10 times...been there done that with a baby and heck we might even have more kids to take in the future. To the naysayers that tell you the baby won't remember, they are correct but YOU will remember you baby in such a special place.
The best advice is know your baby and his or her limitations...if they have colic at home do you want to share a hotel room? if they get car sick, do you want to drive? if they get ear infections do you want to fly? You know your baby better than anyone, plan your trip to thier needs.
One baby is very easy and you will be surprised how many attractions you can enjoy with your baby. On the attractions you can not take your baby you can get a baby swap pass. Do a search on here and you will find oodles of info her about them.
Take advantage of the baby centers in each park if you little one needs a break. A fussy baby can make you leave a restruant, party etc but leaving a park is a pretty time consuming process, especially MK, so try the Baby Care center first to clam he or she down.
Take an extra onise for the protein spills
Take double the diapers and wipes you normally use in a day
Take a hotel towel to line the stroller with and perhaps use later to block out the sun for a nap, to lay baby on to take a stretch or sit on for a parade etc.
Invest in a good stroller fan, ours broke on our 2010 trip and I forgot to get one for 2011 trip. We bought one in EPCOT that was AWESOME and the same price as Babies R Us with a much cuter Mickey design. Our batteries ran out in AK and when I went in to a shop in Africa to buy more the replaced them for free!
If your stroller doesn't have good sun coverage get an additional sun shade, ours is from Sunshine kids
If nursing, try doing on a longer dark ride like HM. It is dark and you are sitting with your DH. Will keep you form stopping later to feed and you can enjoy a ride at the same time. That being said I have a great hooter hider and have nursed all over all 4 parks and most don't notice or stop to tell me they wish they would have invented a hooter hider...
Find Snow White to give your baby a big red kiss, my babies have these pictures and I love them
Use your sunscreen on your baby before you go so you know it doesn't bother your babies skin.
Wash your babies favorite blanket 2-3 days before leaving so it has that home smell for comfort in new place
Baby sunglasses, if your baby will keep them on, my kids did and I think it might have helped...same with a hat.
Since you baby might be walking by your trip make sure you check her feet to see if the shoes are giving her blisters or hot spots
Take the smallest stroller that your baby will be happy in and a baby carrier your baby likes. We park the ours in stroller parking area in a land and then use the carrier between attractions in that land.
If your baby is antsy during sit down meals than avoid them in WDW
Take an afternoon nap in your room, you all might need the down time.
If your baby hate's loud noise, avoid fireworks or cover thier ears
The TTA, Rafiki train, MK's train, HOP are all great nap places for you and baby...try them
Have a handful of gold fish, puffs, or your babies choice snack in your pocket at all times for a good distraction.
Check out ETSY for some darling baby and kid sets for the park...they make great picture and the characters always make a big deal over them.
Have a back up pacifier...one might get thrown in to the lake at EPCOT, just saying...
Enjoy and chill, our trips to WDW with babies and kids in general are great if you just go with the flow...
People think we are crazy when we tell they WDW is the only real vacation we get with our four. We don't cook or clean or do dishes and our family has fun all day every day...so I get needing this trip!
Post some pictures when you get back.
Blessings for a healthy baby