With just one line all you have is a noodle if you it is in the wrong direction.
It doesnt work in every situation like this. I agree with evrything which was posted, but in a scenario like our slip the best line to grab is a single line which is permanently set up at the coorect length on the middle piling in our finger pier. This piling when the boat is actuly finally docked lies slightly aft of amidships. This single spring cant act as a noodle and prevents direction fore and aft. Also much easier to only have to grab on line initially than try and balance two IMHO.
By grabbing this permanent spring line first and dropping the loop over the midship cleat you have eliminated forward motion to hit the dock or rearward motion to drift out of the slip. We also have permanent spring lines on the forward and rear pilings which are much longer lengths whic get applied later as part of the final tie up.
Again the main thing here is to get captured and stop movement. There are many differing ways to accomplish this and somke depend on current, wind and whatever your particular slips nuances dictate. There is no one absolute correct way. In YOUR OWN slips you can do things permanently to make things easier.
Comming into a slip with no lines is always a challenge. As Chuck said getting lines aorund the first poles you pass is a good start and you can use these in combination with your engine to prevent hitting the dock or drifting out while you get the next lines. Getting a line to a midship cleat is usually the great stabilizer if you can also get that line to a piling amidship also.
Practice is the nest teacher so when you are faced with docking in a strange slip you know how your boat handles, and if you have help howto direct that person. This by the way should be talked through BEFORE you hit that slip like agreeing on wind direction current etc. We travel a lot in The lI Sound etc and getting a slip or vgetting fuel is common in strange places. Its good to stand off a minute and acccess the situation with your partner and get your plan together discussing the variables before just charging in.