Before you begin, read the play you are doing your acting piece from.
Once you understand the plot, make some notes on your character and their background, their purpose in the acting scene, their intention. At the end of the acting scene has the character got what it is they set out to achieve? identify what sort of person they are and how that reflects the way they sit, walk, talk, react. Think about what time of day it is and what season.
Decide on your setting. Where does the acting scene take place? Is there any furniture, if so what and where? Draw a diagram to illustrate this.
Is there anyone else in the acting scene, who, and where are they standing or sitting? Make sure your eye line reflects their positioning.
Once you have your furniture (set) in position work out your moves. Try not to move for the sake of it, only if it serves a purpose. For example: The character leaves her chair to cross to a table...why? To get something, to move something or to put something on it. Not to just turn around and come back again. Think how we behave in real life. Acting is being believable in unbelievable surroundings. For example infront of an audience of 450 people! Or a film set full with 40 film crew.
Once you have ploted your moves run through it until it feels completely natural.
Once you are happy, practice. Remember sloppy learning of lines will come back to haunt you at auditions or worse on performance.
Lastly hold your final position for five seconds at the end, that way people know you have finished. Directing your acting scene can be very enjoyable, if you have a colleague or friend who is in the acting business show them and get some feed back. Feedback is very helpful, remember you don't have to feel 'got at' when someone gives you feed back, embrace it in the spirit it is given. Actors are vulnerable and we thrive on praise, criticism is more difficult but very useful.