1) Better all around if you do these lengthwise on a bench. Can wrap the legs around the lower end of the bench when beginning to use heavier weights. It is also a much more stable and solid position.
2) Me (and most guy's) lay the bar on the floor, behind their head. This is much easier with an Olympic bar and a pair of 45 plates on the bar, because of the height of the bar being raised. Than arch the upper body up a bit and reach down behind you, grab the bar and pull up and over the head. If you have a lower bench, than so much the better. You will get more flexible in the reach behind the head part and the full stretch will help you become even more flexible. Most BB'ers aren't as flexible as they think they are, in most exercises.
3) When you begin using heavier weight, the lats will come into play stronger. Breaking into the exercise, with lighter weights, may involve the triceps more. There is a great full stretch on the lats when lowered behind the head. So try focusing on a full stretch when learning this movement. More serious weight really will focus on the lats very hard.
Probably better to work with a higher rep range, like 10 to 12 reps or even more. You will see the benefits after a while. Yes you can build muscle mass with higher reps, even with this compound upper body exercise. It can leave anyone breathing hard after one good set.
When I use a pair of DB's, with the bent arm pullovers, I can clean them from the bench, or the floor, and lay back flat in one motion. The DB's are on my chest, to begin the pushing off the chest to behind the head. All pretty simple once getting use to the exercise. Good luck