Seems like once a year I find myself towing someone in. Hope it makes for good karma.
Like to use their tow line so that when I drop them off at their pier, I can just pass them their line and be on my way. Size of the line depends on the size of their boat; for something up to a 26 foot boat, ½ inch line should be enough. For the length of line, just try to put their boat back far enough so that it will not impact your boat or engine. If you want to get fancy, you can try to put their boat on top of a wave while your boat is on top of a wave, that way the line will have a steady strain instead of alternating between strain and slack and increasing the risk of the line parting. With waves less than a foot or so, this isn’t so important.
To my way of thinking, how you tie the line off is very important. I take the line and wrap it around the body of the cleat at least once before I start making the “figure eights” around the horns. If you start off putting the line around the forward horn of the cleat, it is likely to raise that horn and make the whole cleat part from the boat.
Once I get the boat under tow, it is slow and steady. Make sure the other boat has his outboard down and centered. Also, I make sure anyone on my boat is on the side of the pilot house that is opposite the line under strain, that way if it parts they will have some protection.