Sort of bunch up the marabou feather in your fingers by stroking it from the butt to the tip. Measure this against the body for length and trim the feather. I like the tips of the marabou to reach the tail, sometimes just a smidge longer, but never past the very back edge of the bend. I find it much easier to trim the feather before I tie it in rather than trying to do so afterwards. I get a much neater head this way.
Some people recommend lightly wetting the marabou with saliva to make handling it easier.
I don't do that, but it's something to consider if you're having troubles. Marabou is not as slippery as, say, Squirrel Tail, but it can be a bit cranky at times. Select some peacock herl for the topping. Here you have some choices. I prefer to use prime herls from just below the eye of my full peacock tail feathers.