I think Regan's answer is the best. It is the most culturally appropriate.

what you said 一人で作るのが不安ので見ていてくれませんか？could be translated as "because I'm not happy making this alone why don't you watch me? or can you watch me?". but you could ask "is it ok to make it this way? or "please watch me so I don't make a mistake" or "can you make sure I'm doing this the rite? or "I'm not sure if I'm making this the correct way can you please watch me? and so on

I am a little worried to do this by myself. Could you watch over me to make sure I am doing it correctly? You can also try: I need a little guidance, would you mind watching me make the drink to ensure that I am doing it correctly?

"Hi, can you look at me doing this coffee and checking I am making it correctly?"

How about something like "I am a little nervous making this by myself, so would you mind watching?" In western culture, often we don't admit to being nervous or anxious about doing something so "Since this is my first time making this, would you mind watching (over me)?" would possibly be more natural.