How to combining items?

[The chaser]

void OnStart()
{
AddUseItemCallback("", "acid", "empty", "Unfinished", true);
AddUseItemCallback("", "Glycerin", "empty", "Unfinished", true);
}
void Unfinished(string &in asItem, string &in asEntity)
{
SetEntityActive("unfinished", true);
SetEntityActive("empty", false);
AddUseItemCallback("", "Glycerin", "unfinished", "To_complete", true);
AddUseItemCallback("", "acid", "unfinished", "To_complete", true);
}
void To_complete (string &in asItem, string &in asEntity)
{
SetEntityActive("Complete_yay", true);
SetEntityActive("unfinished", false);
}


Feel free to add screen efects so it doesn't look fake.