Work out how long you would like your tie to be.
For this dress, I made the tie 20cm longer than my waist measurement. I wasn't totally satisfied with this length, and feel for a nicer bow it needs another 10cm. So a good place to start, I think, is your waist measurement plus 30cm.
Divide this measurement by two and draw a horizontal line this length (as we will only need to make half the pattern piece).
Work out the width of your tie. Remember, it will need to be slightly thinner than your waistband, so that it can fit through the buttonhole when the skirt is "wrapped."
For my 4.5cm waistband, the tie was 3cm wide.
Draw vertical lines at with end of the line, DOUBLE the chosen width of the piece (as you will be folding the piece in half to make a tube).
Create a rectangle by joining the endpoints of the lines drawn in the previous step.
To complete the pattern, draw a horizontal line, through the centre of the pattern, and mark as a "fold line."