There are lots of different ways to transfer embroidery patterns -- carbon paper, tracing directly on the fabric using a light box or window, drawing freehand, use an iron-on transfer, use the "prick and pounce" method, print/draw/trace on stabilizer ... I could go on.
For my anatomical tea towels, I started by tracing with pencil onto the towel, but quickly discovered the Glad Press 'n Seal method. I came across a mention of using Press 'n Seal plastic wrap on Craftster a zillion years ago, and can't remember who mentioned it (let me know if it was you!), and this combo of tracing and stabilizer is my current low-cost DIY favorite. The plastic wrap sticks to your fabric, and works as a stabilizer. It's also thinner than a lot of water soluble stabilizers, so your stitches are nice and tight when you remove the plastic. It's more of a pain to remove than water soluble stabilizers, but it's more durable than plain tissue paper and you can also trace the pattern directly from your book, original drawing or computer screen! No printing or backlighting required!
Press 'n Seal plastic wrap
Permanent felt-tip markers (important to keep the marker from poking a hole in the plastic and marking up what you're tracing)
Tweezers and embroidery scissors
1. Find or draw a pattern. I did an image search for diagrams of each of the organs I was embroidering and used those.
2. If you're using a computer, enlarge the image until it's the size you want for your work. Now you can either print your design, or if you're feeling brave, just move directly to step 3.
5. Apply the plastic wrap to your fabric (um, it's probably best to iron your fabric first ... ) and place in your hoop. You're ready to go!
6. When you finish, make sure to remove the plastic carefully. It pulls away from heavy stitches like stem stitch really easily, but for looser stitches like French knots and blanket stitch, be careful. You might want to use embroidery scissors to cut it away from the stitches. For small areas, like the trachea above, you'll need tweezers to help you remove the plastic.
7. Wash your work according to your usual method to remove any adhesive residue.