Hi Lintukoto, i had the same problems when i bought and tried my first pack. The diaper seemed to be enormous, i was trying to diaper myself up laying down on my bed and i couldn't get a tight fit, no matter what, the nappy fell down every time.
But everything changed when i learned to do it standing up. Just press the back of the diaper against a wall or door to hold it while you tape the bottom tapes first. You have to tuck the front wings tight with your opposite hand while you doing it, and ensure the front of the diaper is around the same level as the back. So if you are tapering the right side first, then you tuck the right front wing tight with your left hand and do the right bottom tape far enough to keep it tight. Then you do the bottom left. Then you do the top tape from the opposite side you began with. So in this case, do the top left and then the top right. This is to compensate, because the first tape you do usually pulls the nappy to that side. Then you can readjust any tape you feel not tight enough or overly tight.
If the tapes from each side are very close or touching each other, then maybe the diaper is too large and you have to bought a size down.
Different bodies have different tape landing zones, but usually you want to tape the bottom tapes pointing slightly up and top tapes pointing slightly down.
For the leaks, ensure that the leak guards are up, and roll your finger all around between your leg and the leg gathers. It has to be a little tight, but not that much. Also, fold the diaper in two along the long side, before you diaper yourself, to make something like a "U" form, so the pee is channeled to the back.
So, that's it, hope it helps! i'm sure there are some diapering guides here on the forum.