Planning and communication are the keys to not being nervous. Assuming you do end up doing this (and not taking Den's advice above) you need to be prepared. First of all, you talk to her alone - your Dom should not be around, really, as if he is there then it becomes you and her serving him not her serving you. You talk to her about what she wants and likes, what she has done before, what she would like to try. Get some ideas from that, maybe do a checklist. Make a list from that of possible punishments and rewards and remember the golden rule of punishment/reward: if it is something she likes and wants then it is a reward, if it is something she hates it is a punishment.

With this in hand you can start planning. The more you plan the smoother things will go. Go as far as making a time linked list of activities

i.e. 1800: Summon sub into dungeon and order them to strip and kneel near to the chair. Humiliate them with some verbal abuse

1810: Put collar on sub and tie hands behind back. Apply nipple clamps

And so on.

You don't need to follow this like a script all the way through but it gives you a basic outline you can refer to if you get lost and confused. Then you can focus on being the dominant persona...