I have a 1957 Mercury Monterey. The left rear door will not stay latched shut. So I need to remove the interior door panel trim. And in order to do that I need to remove the interior door handle. How do I do that? My manual says...

"Remove door inside handles. This is accomplished by pressing trimmed panel inward to permit insertion of tool 21812-A between handle and escutcheon. Turn tool to release spring retainer."

Would kind of tool do I need?

Try this

http://www.shop.com/+-a-Ford+Door+Handle+Removal+Tool-g1-st.shtml

Thank you. That is the tool I need.

I tried what Ron of Chicago said in his reply to jumpinjivinjoe and had no success. Ron can you advise, please? I inserted the tool at the handle end and rotated it upwards to 90° and beyond to nearly 180°. The handle would not come off. Is there a certain technique required? Thank you.

The handle has a "C" spring with ears, on the underside. In fact, window cranks AND door handles have the same setup.

You should simply slide the tool under the back, and catch the ends of the "C". When you do, keep pushing straight in. You are trying to spread and push the clip out the opposite side of the handle with the tool. Twisting will only get you in trouble, and it will damage the spring, (making removal MUCH harder). There is a learning curve. Sometimes you will get lucky. After doing a few, you will visualize what's happening behind the handle, and they get easier. Remember, the tool works by pushing on both ends of the clip, at the same time. It spreads the clip, and pushes it out of its groove. When that happens, the handle should pop off the spline. Save all your parts, and don't damage the door panel.

You can get extra springs everywhere, cheap.

simplyconnected is correct.

The clips can go on one of two ways and they are 180 degrees different. Sometimes you can push the interior panel in far enough to see the clip so you will be able to tell how it comes off.

This is what it looks like