Does anyone know how to make a 8 foot palm tree?

Make a half real one.

It has to be stable, so for the base use a cross base and upright from an upright fan. Or the heavy base from a standard lamp would do, with the lamp fittings removed.

Go to your local fabric shop and ask them to give you a number of cardboard rolls that fabrics are wrapped on.

Pull a small roll onto the fan base, that will just be a squeeze fit on it. This gives you a bit more height. Add another slightly bigger onto it for more height (you will have to squash the top of the first roll to get the second one onto it, and wrap it tight with duct tape).

At this point you have about 6 or 7 ft height. Put two much wider rolls over the base ones, to form the trunk of the tree. These would be about 4 inches across, and too solid to bend. With a strong knife (or small saw) cut the top roll down into 8 segments, cutting down about 12 inches. Fold down every second segment to make 4 slots. Squeeze 2 real palm branches into the slots and tape up the flaps to hold them tight.

Cut rolls of brown crepe paper into strips about 12 ins wide, cut one edge of each strip to form Vee shapes. Wrap the strips around the trunk so the points between the vees curl out, to represent the bumps where old branches have fallen off.

To get real branches, talk to your local parks maintenance department, if the parks have palm trees in them.

If they do not, you need to make your own branches. Use the fabric rolls that are soft enough to bend a little, to be the centre spine of each branch. Squeeze the rolls to be thinner at one end. Use white glue to glue two wide sheets of green crepe paper to each spine, so the spine is between the sheets. When it is dry, use sharp scissors to cut the paper up into "leaves".

Poke the branches into the trunk slots, same as for the real branches.

The cardboard ones are lighter than real ones, so you could also cut some holes a little further down the trunk and put an extra layer of branches in.

Do you need it to be 3 dimensional? Then the first answer is a great one. But if you don't, and it can just be something attached to a wall, then here's what I did last week (on a smaller scale).

I bought a roll of brown craft paper, usually available and shipping/mailing centers or office supply stores. And I bought a lot of green card stock/poster board.

I just took a length of the brown paper and trimmed one end to be narrower, like the top of the palm tree's trunk. Then I cut out palm-frond shapes out of the green card stock/poster board and attached it to the top with glue. I even cut "coconuts" out of the brown paper and drew on the holes that coconuts often have on the bottom, with a black marker; and attached them in the "leaves".

For extra effect I took brown paper bags and crunched them up and then flattened them back out and cut strips from them that were the width of the tree "trunk", attaching them with glue, which made it look like the sections of the trunk where old branches had broken off.