Kite flying to one side, is asymmetry in front lines, mixer, bridles or canopy. To correct it you address them in that order till the problem is tolerable or fixed.
Front lines. I have had a number of uneven lines that can only be explained as having been made that way. To adjust lines you first need to take any shrink out of them. Applying a large even force to each line seems to work well. You can connect both lines in a chain so you can use your full body weight and the force is even. A small rope run through a pulley with 2 lines connected to each end of the rope, the other ends to something solid and even, will allow you to apply even tension at different lengths. Sharpie marks made at the same point/s on each line close to the pulley end, this can be used as a guide to adjust the lines later inside.
Mixer. With the mixer locked together at the front line attachments. Check for asymmetry and adjust or stretch out. Once even continue high up the bridles.
Canopy from foilzone
Great article written by Peter Lynn explains it http://www.peterlynnhimself.com/Pilot_Tuning.php
. The bit to focus on is the sewing at the end. The diagrams dont match the explanation. Assuming the front lines and A bridles match, and the A tow points match, proceed to sew.
How I did it. Starting at the opposite side/end cell the kite turns to. At each cell line on the top skin fold the kite in half. I used one hand to hold the leading and trailing edge at their extremes. I then placed a finger in the middle to find the center of the cell wall. Use a pen that will soak through the cell and place a dot on the top skin near cell wall. Do this for a few cells in succession, the more the worse the turning problem. Hunt down the dots inside the kite through the the cell holes. Starting at the end cell. Sew a one ripstop square width pleat in the top skin,from cell wall to cell wall. Leave a 2 cm relief on each end, i.e. sew a diagonal on each end of the pleat so the pleat doesn't start straight at the cell walls but gradually. Use the ripstop squares as a guide to sew an even straight line. Do this to a few cells and subtract or add more later or one at a time. If the kite is unflyable you'll need about 6.
There is other possibilities of doing bigger pleats, pleats at other places than 50% and multiple pleats per cell. I would guess changes to the end cell/s have the biggest effect. This will also make the kite more stable. If applied to both sides it would reduce or stop tip flap, depending on the amount done.