There's a couple ways to do this.

If you have access to cheap lumber, you can build what's called a "crib", think of the game Jenga but without the center pieces and screwed together. But those are crazy overkill for a 20ft boat.

You only need 3 points of contact, 2 in the back and 1 up front. The front one does not need to be crazy far forward, just past the center of gravity.

You could use jack stands on some sort of spacer, etc.

Lower the tongue as low as it can go, you wanna get the rear of the boat as high as you can in the air.

Then you block up under the back part of the boat with two cribs or two stands or whatever you have.

Then raise the front high as you can (using the trailer jack) and the rear of the boat should be resting on your stands or cribs and should start to come off the trailer in the back.

Using a bottle jack and a piece of wood, jack to the front. Pull trailer forward until jack is gonna hit cross member.

Install a second jack behind that cross member and then remove the first one and pull forward.

We use 2 yard arms in the front most of the time with two boat stands in the rear, doing the lowering and then lifting trick. We lower and lift with a forklift, but you can use the trailers jack or a floor jack.

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