Just make a player variable like playerprogress, set it to 0 to begin with.
Set it to 1 when they're on step 1, 2 on step 2, etc.
Make the NPC only do what it's supposed to when they are on the correct step by checking the variable. You can also use this to make sure they only get a reward once.
The code for step 3, for example, would look like
If (playerprogress < 3) {
mes "You dont have business with me yet.";
close;
}
if (playerprogress == 3) {
do whatever step 3 is,
give the reward,
then set playerprogress to 4 so this part cant be repeated.
}
if (playerprogress > 3) {
mes "I'm done with you! Go see the next guy!";
close;
}