For making cordage, from what I've read, you need to let them dry out (keep them in as long lengths as you can)
The backstrap sinews will dry out to form thin ribbons - the leg tendons will form hard round 'dog-chews'. The way they are processed from there is slightly different..
for the backstrap sinews, use your finger nail to divide the sinew across the width. Splitting it down further and further until you have the thickness you want.
The leg tendons will need to be seperated out into fibers. This is achieved by pounding using a smooth log on a flat stone or piece of wood to make the fibres seperate - then pull them apart to the thickness you want. Make sure that what you are pounding with / on is smooth as any sharp edges / roughness will tend to nick the sinews - making them prone to breakage.
I've only had limited experience of making cordage from sinew, so will be watching this thread for more info