I had an old top which was falling to pieces and rather than throw it away, I decided to unpick it to use for pattern pieces. You don't need to unpick anything of course as if the top is simple, you can still take a pattern from it. I'd start simply though with something like a tee shirt rather than a complex dress!
Now, any how-to no matter how carefully written down can appear like gobbledygook when glancing through, which if it does, I apologise!! It is in fact quite a simple process, and you might find it easier to google and follow some the the excellent videos on youtube.
Here's my cotton top carefully unpicked to give me a sleeve, back and front bodice pieces and some bias binding that finished the neckline.
Iron the pieces so they are nice and flat, and for the front and back pieces which in this case will be cut on the fold, iron in half matching the shoulders and sides carefully.
Lay the pieces under a piece of pattern paper and carefully trace around the shapes, noting where folds are and labelling each piece of pattern so you can identify it easily. (It's also useful to mark a straight line so that you can place the pattern pieces on the grain of your new fabric when cutting out)
Now scroll down a bit to follow the making up.
If you haven't unpicked here's a couple of extra steps.1) If you are not unpicking, then look at your piece of clothing and decide how many pieces went into making it. Usually there is a front, back, sleeve and facings. You'll need one pattern piece for each.
2) Turn the garment inside out and lay it on top of the paper. Start with the sleeve. Smooth it out so it's lying flat - you might find it helpful to pin into place. Draw round the outside of the sleeve, noting where there is a fold (if you've unpicked you will be able to just draw around the piece) Look at the seams and draw carefully around these noting how much seam allowance there is - you'll need to add that later. Look at the hem - measure it if you need to - and mark the edge of the fabric and then add a dotted line to show where the fabric would end if the hem was folded out.
3) Pin the sleeve to the paper if you haven't already done so. You''ll need a pin at each end of the shoulder/sleeve seam. Carefully fold back the sleeve, and draw in the armhole seam.
4) Note how deep the neckline fold is (on a tee shirt). Fold the front in half along the centre - pin if it helps, and draw round the shape. Unpin and mark a seam allowance (not the centre fold) on the edges and include the extra at the bottom for your turn up.
5) Do the same for the back.
6) Remove garment from paper, and draw straight edges with ruler, and even out any uneven lines, adding a seam allowance where necessary. This can be what you want depending on whether you're using a sewing machine or an overlocker.
7) Cut out the pattern from the paper.
You now have your pattern pieces and need to cut them out. Make sure you mark any seam allowances. If your garment was overlocked, then these might be very narrow and now is the time to make adjustments to suit you and your machine.
Lay the pattern pieces on your new fabric in the same way as any other pattern. You won't have detailed making instructions of course, but will know what bits need to go on folds and what pieces have to be cut out as a whole.
I found that marking hems etc with tailors chalk helped.
I made up the sleeves first, by sewing the hem line. I used 3 lines of stitch for decorative purposes. I then sewed the seam line. I don't have an overlocker so used a straight stitch and then oversewed the edges with a zig zag. My seam allowances were very small!
The back and front were sewn together at the sides and the sleeves fitted in the normal way. The neck line was finished with bias binding on the original, and I had to buy some as I don't have a bias binding maker.
I pressed the bias in half, and sandwiched the edge between, before stitching. I then turn the whole to the inside and stitched again through the binding and edge to make it lie flat. I used 3 lines of stitch to match the sleeves and hem.
The finished piece. I didn't want to use expensive fabric until I was sure this would work, but now have a pattern I know fits and I quite like, so can run up another at any time!