Suburi is solo practice with a weapon in the form of repeated strikes!
Practicing suburi is excellent advice.
Do it all the time and get guidance from Sensei on how to strike precisely, using your hips.
These are many good tips on how to execute ikkyo.
Remember that timing is very important. If you enter to late you will struggle if you enter to early you may lose your balance or change the attack.
I also think a great deal about entering. There must be some element of irimi to unbalance an overhead attack.
Practice ikkyo against a sword attack from jodan. Practise slowly.