Lift the needle to the highest point by turning the handwheel. Lift the pressure foot to release the tension on the system. Place the spool on the spool pin and put the spool cap on to hold the spool firmly.
Next bring the thread to the thread guide and snap it into the guide. Then slide the thread under the guide and through the slot (use one hand). Next lead the thread down on the right of the tension knob and up on the left. This puts the thread in the tension discs. Next slip the thread into the take up lever. (Bring the thread to the top of the lever and back towards the machine. Then slide the thread into the slot on the take up lever and then back towards you.
Next lead the thread through the next pair of guides. One is on the machine below the take up lever; the other is attached to the needle holder.
Finally thread the needles from front to back.
First check if there is a slight drag on the upper thread when you pull it through the machine (pulling it under the foot of the machine). Move the tension knob to the right to increase the tension. When you stitch a test piece, your top thread is visible on the bottom of your sewing thread, rotate the tension knob to the right by half a turn (get the dot closer to a higher number).
If there is no change in the tension as you turn the knob, take the machine for service. The springs in the tension system may need to be replaced.
The manual is available from Singer here: http://www.singerco.com/accessories/manuals.html
. Enter only the 2517 model (leave off the c). A pdf is available for free; there is also an option to order a hard copy. A picture of the threading route is on page 28 of the pdf (page 26 as the manual is numbered).
I hope this helps.