generally speaking, when the bow of the boat is touching the forward section of the bunk boards, the bow will center itself with a little help from the outboard. Keeping the rest of the boat centered on the trailer then comes next. If the area is shallow, it will create some difficulty to loading up and same vice versa if the ramp is too steep. Another is of course if the ramp incline is not level or leaning to one side is another issue and it is that when many folks have a difficult time.
Once I was at a marina when the lake was flooded looking at an owner teaching the new buyer how to get the boat onto the trailer. The ramp was tilted and every time that the boat was line up what to appear be perfectly centered on the trailer, the boat came out cockeyed on the trailer as the trailer was leaning to one side. After about 45 minutes with both parties tired and worn out, I explained the trailer was not level causing the issue. I told them to put the boat onto the high side of the trailer and as they pulled the boat settled perfectly onto the bunk boards. The other issue was the ramp was steep to some degree and they could not understand why the bow would not nuzzle into the bow support perfectly on the trailer. I explained when you pull the trailer out, the trailer winch stand bow support in a sense goes down and away from the bow of the boat. Not a problem. Once the mast was down, I showed them on a level parking lot to move forward with the trailer, brake, tighten the winch strap doing this several times until the boat was all the way up on the trailer. He then asked who I was.
I spent another two hours going over mast raising and lowering and tying down. They understood not to put a tie down strap over the bow of the boat but only in the rear and turning the strap a couple of times to prevent wind slap. In addition, I explained tying the rudder up with an additional line for precaution and making sure the safety chain on the trailer winch stand was attached to the bow. At that point, I had a long drive home.
To my surprise was a check in the mail from the new buyer. To his surprise, it was returned voided with a note, help from another sailor and friend which payment is not necessary.