|Making a ruboff is a good way to take the pattern from a ready to wear garment without unstitching a thing.
First you have to establish a grainline, (too bad not everything's a plaid, then it would be easy) both horizontal and vertical. I do this by pinning a row of pins along the center of the particular panel I'm working on, and then extrapolating the horizontal grain by peering closely and guessing a lot.
Hint: to pin more securely, put the pin thru the fabric twice, take 2 bites, so it looks like 2 dashes. Pins are less likely to fall out this way.
Then you have to mark (in pencil) the exact same grainlines on your chunk of muslin thats approximately the same size as what you are rubbing off. (If you work with a piece that's too big it can hamper you.)
Pin the muslin along the garment on the marked grainlines
Then smooth and pin your muslin out to the seam lines.
If you have a dart or ease, smooth and pin below that spot, and then upwards around it. The width of the dart will emerge, and then you can pin the dart closed on your muslin. Similarly for sleeve caps you can find that 1/2'' of ease to pinch out front and back, or how much gathers are there and then smooth and pin to find your armhole.
To do a sleeve properly, you need something to put inside it. A rolled up towel will do, or a sleeve board possibly.
A lot of people use tailors chalk at this point and just rub it along the seamlines to mark the seams. That's why it's called a ruboff. I prefer to use a pencil myself, as it's much easier to control your markings and get subtle differences of shape. You can get buttonhole placements, all the details of your original this way.
Afterwards, unpin and lay flat. Use a ruler and curve to "true up" your lines.
Add seam allowances and use directly as a pattern, or use your tracing wheel and transfer to paper if you want to fool around with it some more.
The nice thing about ruboffs on muslin is being able to baste or pin it together and try it on. Also you can have enormous allowances and add for alterations in advance, or at least have the extra in there to be able to adjust.
I usually add my needed adjustments on to the pattern at this stage. I also sometimes end up needing just a wee bit more fabric, so I pin or tape on additonal bits of muslin as needed.
I keep my rub-off patterns in big manila envelopes with a quick drawing or description to remind me of what it is.
You can use parts of various ruboffs to create entirely new garments. You can also pin them together and work on a dressmaker's form to adjust the fit or style.
If you do this often enough, it's a good idea to buy an entire bolt of muslin, then the price comes down enormously and you loose the fear of 'wasting' fabric. Actually you can use anything to make a ruboff, but muslin is good as you can see your markings easily and its cheap. It even works for stretch garments, you just can't try them on afterwards.