You can go ahead and clean you're fretboard and get all that dirt of the guitar when you have the guitar strings off. It's much easier and if you look close at the frets you probably will see some dirt that's collected over time. This is skin oil, skin and dirt that you have there. You can use some fretboard cleaner and polish to get that nice look back to you're guitar. If you don't have anything handy you can do it afterwards also, but it's much easier to do when the strings are off the guitar. Don't use water when cleaning or any other household cleaners. Some say you could use citric acid, but this will dry out the fretboard. I have used a little cooking oil or olive oil on my guitars fretboard. And it worked very well. But I would not put anything else on the guitar, because the wood will soak up anything and if done wrong this will damage the guitar.
Put the guitar strings back in the right order. The thickest gauge guitar string is called E-6 this one is the first string you see when looking down at you're guitar when you are playing. And then it goes down to the thinest string that also is a E string but called E-1. So the order is E A D G B E. Have a look at the picture above and you can see how it should look if done right.
Putting the strings back is pretty easy, put the stings back in the tuners and turn you're tuning pegs. Try and get the guitar strings to lock in place so that they don't slip. And try and get the winding to look good. If it looks good it will sound good. And also give more life to you're new strings.
Tommy Emmanuel - String Talk