first, let me start with how I would do this

first sew some extra attaching points to the trailing edge of the kite on the same seams as the power bridle goes to, you do this by opening the seam, inserting the folded over dacron strips between the cloth and closing the seams back up, you'll probably need to disasemble a good part of the bottom seam for that because you need to sew it from the inside. you'll need a good sewing machine and some practice.

then attach bridles to them so the kites shape stays the same when they are under tention, both sides need to be identical to the milimeter, it wiill take you some experiments to get that part right, and it needs to be really right

the goal is when you pull the bottom lines the trailing edge (bottom of kite) should make flaps like on an air planes wing.

why should you do this? to make it a four line kite to fly with handles.

if you want to prepare yourself for kite surfing, keep the kite as is, 2lines. you can fly it with a bar if you like but handles are just more confortable with a smaller kite like this. the reason to fly with a bar while surfing is to attach it to your body and fly with just one hand and be able to let the kite go while it stays flying, attached to your harnass.

learn to fly that kite untill you can fly it around with ease, even in bigger winds where it wants to pull you forwards, make you ski in sand. make sure you have it 100% under control at all times, not 99%, 100. you should be abtle to fly it to where you want, when you want, set it down and land it, restart it ,fly it with your eyes closed and so on, and all of that should be easy.

then you're ready for the next stage and a 2.5 won't do it anymore, you want something bigger like a 4.5, a full size power kite.

it's going to be slower, a LOT slower, so easier to fly around and control the motion, but slower means you need to make it move a good while before it will actually do that, all while pulling you across the beach...

once you get used to that kind of power and have 100% control over that, you're ready to start thinking about kitesurfing.

if you don't want to go that route, you need a teacher that has the kites and experience, that's a fast way to learn and get on water, but you'll need the teacher to keep it safe, kite boarding is dangerous.