Its definately something that gets better the more you do.Personally I would say remove the majority with wax then define with tweezers.The reason being this,the heat from the wax opens the pores and makes the tweezering easier and less uncomfortable for the client.
I have clients who hate me using the tweezers(I am one of these)they find it quite painful,so I wouldn't ever just go right in and do most of the shaping with tweezers then wax , when somebody is booked in for waxing of the eyebrows.
I prefer to be waxed anytime over tweezering and would be most unhappy should I go to a therapist and she started to tweezer my brows when I was booked in for a wax(obviously I do my own but when I do tweezer odd ones I find my pain thresh hold is not very good)This is why I say do your main part of the brow shape with the wax first.(I always tweezer afterwards and tweek the brows into the final shape,as this shows a conscientious therapist imo)Also I always book clients in for 15 mins.
I used to get very nervous when I first qualified,we all did i'm sure,it's all a matter of time,now I thrive on seeing a mono browed Dennis Healy double because I know I can do a fab transformation.
Just take your time,best not take too much off in one go,dont re use the same wax strip,this way you wont go over hair you dont want to wax off with old wax.You can put vaseline over hair you dont want to wax off but this is a bit of a pain tbh.
I use the Clean and Easy roller wax system for the bulk of my waxing but pot and spatula for eyebrows and some other waxing, and though it does come with a small head for eyebrows I would never use it,I dont think you can get a very precise shape.I use a snapped in half spatula but you can use an orange stick for a very precise application.
Practice is important,the more you do the more you will become confident and then you will actually start to enjoy doing it as I do.
Remember if the client has any defects when you are carrying out the consultation,i.e bald patches or irregularities,to show her/him this in a mirror before carrying out any waxing and write it down on the consultation card with sig,that way you cant be blamed for this.Also if the client has a lot of white hairs this can make the end result look uneven so a tint would be a good suggestion and even them out.HTH a little.