I design my instruments in an older version of AutoCad (AutoCad LT 2003) and print out my patterns myself in a normal desktop inkjet printer. The trick is to establish a set of gridlines on the drawing so that you can print out a pattern in multiple sheets, and get them lined up correctly.
For example, a body will normally have to be printed out in four sheets. In AutoCad, I draw a grid of red lines over the body that break it up into four 7" x 10" rectangles. Print out the four sheets, and each has the red rectangle of grid lines on it. Trim them with scissors and tape them together, lining up the red lines.
Tip: When I print patterns, I print them on thicker 28lb paper (called cover stock or card stock), rather than normal paper. The card stock will feed better through the printer, usually giving you better accuracy along the long direction. Thin paper will tend to slip along the feed direction and cause inaccuracies. Inkjet printers are surprisingly accurate, in my experience, if the paper feeds correctly.
That's another reason for the grid lines. If you draw them in your Cad program as exactly 7.000" x 10.000", then you can use them to test your printout. Put a good ruler on each sheet and verify that the grid lines are exactly to dimension. If not, print it again, or find the problem.
To answer your other question: If you're starting with an undersized pencil sketch, and want to get to an accurate Cad-generated pattern, here's the process:
First, scan in your pencil sketch, which will give you a .jpg image file. Then, in your Cad program, start a new drawing and import the jpg file into it. There's usually a function like Insert Image, or something. Then, in the Cad program, draw up a framework of lines on a new layer to establish the dimensions that you know. For example, if you're working on a body shape, draw in some lines to show the length and width that you want it to end up. Then, move the image file under that framework of lines and stretch it, scale it, or expand it as needed to fit those dimensions. Finally, you redraw the curved lines in the image, drawing new Cad polylines right over the lines in the image. Then you can shut off the image file layer and go to work adding details to your Cad file.