How to alter a hem with a slip stitch
To alter the hem on a pair of trousers, the first thing you will need to do is measure the desired amount that you wish to either remove or turn up from the bottom. To do this, the simplest thing you can do is source a few safety pins and fold the current hem outwards drawing it up towards the waist line and safety pinning where you would like the new hem to appear. This will be the folded part nearest to your ankle. The best place to stand is in front of a mirror so you can clearly view what the new trouser length might look like. When you have secured a few safety pins in place make sure you sit down, get a sense of the new length and most importantly, if you are intending to wear particular shoes with this item you are altering (particularly ones with heels) make sure you have these on at this stage so that you get this measurement right. Leave the safety pins in place and remove your garment, setting it aside.
If you are intending to remove a portion of fabric to shorten the item, you will need to measure the amount you have folded over. Make sure you leave enough fabric to create a hem (this is the turned in portion that will be secured on the inside) with the addition of a 1.5cm seam allowance that will be tucked in to make it nice and neat. You can cut off the excess.
You should now have a section of material that you will fold inwards and secure. It is often easiest to turn your garment inside out and neatly press this new hem fold into place.
To fasten a hem you can use a machine, but it is often practice with wool or more delicate fabrics to create a discreet slip stitch to secure.
Once your hem is neatly pressed and you have also tucked in the 1.5cm seam allowance to the top edge (fig 1) you are ready to stitch. For finer fabrics, the use of a thin dressmaking sharp (needle) is best as this will allow you to pick up just a few fibres on the inside of the garment, without puckering or holes on the outside.
As in fig. 2 the slip stitch is a small stitch that starts on the inside picking up just a portion of fibres, not wishing to push your needle through to the front. You will then aim your needle towards the folded edge (where 1.5 seam allowance has been tucked in) and draw your needle through the very top edge of the folded fabric and then repeat the process in a continued line.
Once you have run the entire way around the hem and all is secure you can then tie off your thread with a simple knot and your new hem or repaired hem is complete.