Hi and welcome to the forum.
Before you start adding more to the tail, try resetting the bridle.
Below is a picture of what I think is close to the kite you have bought.
If so, then you will only need a Sharpie or Texta for marking the lines.
Before going out to the field, undo the bridle knot (where they all come together).
Tie an overhand knot in the end of each line - this makes it easier to handle when adjusting in the wind.
Find the line to the middle Keel and use the Sharpie to place a mark about 30mm (Inch and a bit) from the knot for reference.
On the field.
Forget about the lower bridle line for now.
Take the Left and Right lines and bring them together with the middle line.
Adjust the two outside lines so the front of the kite is straight (not curved away or bowed towards you).
If the kite spins or leans to the right, bring in the left side and let out the right side lines until it is stable.
Do the opposite if it spins/leans to the left.
Once the kite is stable, mark the two lines at the middle line mark.
Then, take the lower line into the group and pull it in until the kite develops power and tries to climb.
Be careful, too much and it will pull like a truck but sink back to the ground.
When you are happy with the performance mark the line with the others, then tie an over hand knot just past the marks (so you can see if any line changes while tying off)
Let us know how you go 😉
