Hi
For the yurchenko, i would spend lots of time working round-off back layouts of a spring board. Focus being on taking off tall, and impacting the springboard more vertical. This is critical for a proper take off/entry into the back handspring onto the table.
Ontop of that lots of time spent spotted and not sport working on standing flics uphill, and learning when blocking to stop the feet and raise the shoulders.
Tsuks - WOrkin on endless round-off entries. Focus is on not turning from the board but rather like a good round-off on floor to turn late. WIth lots of prop to stand onto stack mats above vault height. Do not turn them to your back until you can learn to prop and stand up.
These are the as basic drills as you can get. THey are also the foundation to any good vaulting. Ontop of that if i were you i would study Cheng Fei yurchenko entry and real hard to indentify the things i am talking about (the standing up..) look at angles, speed, feet, knee positions ect.