The Easy Part: Buy it ready-made from a reliable butcher.
The Hard Part: Doing it yourself!
Buy intestines, hearts, livers, kidneys, spleen etc. from a very reliable butcher. Many sell them already washed. If you wash the intestines yourself, place one end on the faucet and run cold water through it continuously (don’t use hot water because the intestines will toughen up). Make sure all the innards are cut to about the same size. Pass them through the skewer. Tying them up with the intestine takes some skill. The intestines have to be tied around them like a spool of thread several times until all the innards are completely covered. Brush with the lamb marinade and roast over a spit.