1) take a good look at the item if it is a shirt be aware you will need the front, back, sleeve, collar and any other section. Pants you need front, back, waistband, pockets. Always start simple you can move up to complicated when necessary. In this case the pants. I needed one front and one back panel. you can either unpick all the stitching or do it the lazy way. in this case I cut the left front out and the back right by cutting into the fabric on the opposite sides leaving me with this.
2) then lay the fabric on paper. My sister is a teacher and had several large writing pads about 24" by 36". I like this paper because it is lined and easier to mark. However any large paper will work. I have used wrapping paper which I bought on sale after Christmas a large roll is better then tissue and easy to story.
once you lay the fabric on top mark the paper 1/4 inch all round making sure to mark where pockets and any darts go, then cut. this leaves you with a nice pattern.
If I am going to use a material that has less stretch I would mark 3/4 to 1 inch around the sides depending on the material. You do not need to do this for the mid-seam and crotch area.
3) cut out the material making sure that you mark which is front and back. Chalk works good for that. I wrote it on the inside until I had it all assembled.
The pants only needed to be hemmed. for pants that have an elastic waist I prefer doing a separate waist band this gives it stability.
Pants with elastic are one of the easiest to do and for those of us plus size I found that doing a longer pocket set in the side means that when you sit down the stomach expands nicely with out being too tight and when standing the pocket looks normal. This really only works when the inside of the pocket is made of the same material I also made them a little deeper so if she has anything in the pocket it won't fall out when she sits another problem for the plus ladies.
this if the first of several pants cut from design that she knows fits her well and is lot cheaper then trying to find a pair.
when you finish the best way to store the pattern pieces is a ziplock bag. I write out the information on paper making sure to notate how many pieces. also on the pattern pieces themselves so that I do not mess them up I will put 1/5, 2/5 etc and the pattern number is the date 111012
Have fun and don't be afraid to practice.