Im no expert, but Ive done quite a bit of cable over the years.
Here is how I do it.
Cut the cable with a torch, while doing so, melt/braze the cable ends solid
Wear eye protection, Heavy gloves and apron. Clear your work area of all flammables.
1.) Burn off the cable, once burned out, bring to heat and brush very well, then I reheat and tap mine on the side of the anvil. The is usually a LOT of crud in cable, so the more you can get out initially the better.
2.) Start heating, before a scaling heat, flux HEAVILY, flux should be running thru the cable. Once the cable is at a welding heat, I take a good grip on it and fling it to throw out as much flux and gunk as possible (do this carefully!!! Watch for fires!!!) Reflux cable and return to forge.
3.) When cable reaches welding heat remove and twist cable with the lays to tighten it up. Re flux and reheat. Repeat the tightening process until cable feels solid.
NOTE: I reflux at every heat until I am sure the cable is solid
4.) At a welding heat I use the corner created on the step down of the anvil and tap lightly into the corner while rotating the cable. Start at one end and follow the strand lays to tighten up further. Repeat this until the cable feels solid (will also notice a difference in sound)
5.) Once Solid I will forge square (flux if you are not certain its solid) then forge into a flat bar.
6.) If you have lost mass in your cable, you can cut, stack and re weld to increase the size of your billet, but this will greatly water down your pattern. It is best to start with the largest cable you can find. I have a stock of 1 3/4 cable I got from a barge company.
Things to look for. Some cable has a rope or plastic core, Dont use this type cable.
Most scrap cable is saved and sold to the scrap yard. Check there, its a great place to find it.
Check with you local crane company. They have to replace the cable periodically, if your lucky you can get all the cable you will ever need for the price of a 12 pack.
Rigging companies are also a good source. The wire chokers used for heavy lifting can be up to 3"!
As I said, this is how I do mine, use this as a guide to get started, and play around and see what works for you. Cable damascus is known for inclusions, so FLUX, FLUX, FLUX! ( I use 20 Mule Team Borax found in the laundry detergent section of most grocery stores) this flux will eat a Kaowool forge liner like cotton candy, use a hard fire brick or some other liner for the bottom of your forge.
Hope this helps, If you have any questions, feel free to ask, I dont know all the answers, but I bet one of the pros on here do!!
Good Luck and God Bless!
Mike