I fettled a longer one for the shareboat as a few of us wanted it long enough to stand inside the doors.
This assumes you still have the old one to measure from.
I measured the original tiller bar for first hole and drilled a pilot hole then enlarged it (don't go too big too soon, i tried to jump from pilot hole to full size bit in one go and it kept grabbing the bit, step up in sizes or get one of those conical step drill bits). I also made a U-shaped box to hold the tiller bar in whilst i drilled to keep it straight as i didn't have access to a vice when on the boat, wedged in with some shims to stop it rotating.
Once i had the first hole done, I placed in situ and aligned using the tiller pin, then drilled the next pilot hole straight through whilst on the bar, then continued to enlarge to correct size.
The hardest part was getting the new handle in the new bar, both came from Midland Chandlers and were allegedly the correct size. Even after copious sanding the fit was so tight i didn't need to use the wedge. That handle is never coming off, ever
Pretty much as @ditchcrawler has said above.
I also had to make a new step-side bracket to hold both the old and new tiller bar as some owners wanted it as was, but that's another story and the joy of shared ownership