There is more fullness in the edge of the fabric than the line the hem will be sewn.
Start by basting with a running stitch all way around the hemline, barely 2mm 1/8″ longer than the finished hem so it will turn just on the edge on the inside of the hem.
You need to very gently gather the excess fabric by basting a thread and easing the fullness around the skirt evenly.
Natural fibre fabrics are much easier to work with here than synthetics, be patient and find the correct temperature to press man-made fabrics. Cotton, silk and fine wool are the nicest fabrics to work with.
Press the hemline with a cloth between iron and fabric. A barely damp cloth will give the best results.