One step (Hanbon Quorogee) is the simplest, and then again the most difficult pre-arranged sparring. Simple in that there is only one step, difficult in that the defender must make an instant decision on how to block and counter attack. The premise is simple enough - a pre-arranged attack is blocked, then counter attacked.
Before beginning, it's important to get the distances right. Ideally both students will be of a similar height as otherwise the distances won't work well. Face each other and measure distance by having one of the two extend an arm fully so it just reaches the other students' shoulder. Once you have done this a few times you will get used to the distance required and measuring up will not be necessary any more.
I don't know of any set sequences for one step - so this example should be taken as just that, an example. The golden rule here is that once an attack and a defense has been used, it should be re-used from the other side, then re-used in reverse (attacker becomes defender) so the same sequence is carried out twice by both students.
From ready stance, fall back with the right leg into long stance and execute a lower block with the left arm. Kihap.
On the ready signal from the defender move forward with the left leg into long stance executing a mid section punch. Kihap.
As the attack comes forward, fall back with the left leg and block the punch with an inner forearm block with your left arm. The attackers arm needs to be pushed across his body towards his left.
His right side is now open to attack, move forward with your left leg into a long stance and strike his mid section with a right hand mid section punch. Kihap.
The blocks in pre-arranged sparring executed realistically. My instructor tells me that if the blocks don't hurt, I'm either doing it wrong or my forearms are already fully conditioned to deal with blocking techniques. Unfortunately, the former is true. After an evening practising one step, expect to go home with bruised forearms. Eventually your arms will get used to the punishment and it won't hurt as much.
The attacker must make his attack "real". Since the defender knows what the attacker is going to do, he should not be in any real danger of injury.
The counter attack should be executed with as much realism as possible, but it is not intended to hurt the attacker. If the distances are right, you should be able to make a full counter attack (with a good loud kihap) and just fall short of striking your opponent.