The same method works for making a handle for an adze, pickaxe, pulaski, or any tool where the head slides up the new handle and jams up on the large end of the handle.

Step 1: Get Wood

Don't forget firewood! Check out your woodpile, I bet there's some good hardwood out there that's well seasoned already. Green wood is okay for a pick handle also. Green wood is actually a lot faster to work with hand tools. It will shrink a bit, but that's not a problem, the big blob on the end will keep the head from flying off. Try your park department or trees blown down in a storm.
Orchards prune a tremendous amount of wood from their trees. They all have to pay people to get rid of their wood.

Hickory and its relatives are the strongest woods that grow in the U.S., which is why they're usually used for tool handles. Ash is almost as strong and I happen to have some. Don't use softwood, it just won't last, and you'll get to make another handle. On second thought, go ahead!

If you're in the south or in Mexico, hard yellow pine is plenty strong although it's officially a softwood. That's why they use it to make ladders and floorboards.
In warm areas like that invasive "ironwood" a.k.a. Casuarina, called "pine" in the Marshall Islands is great for tool handles. Shape it green and wet, it's very easy to work. Then throw it in salt water until it sinks. It'll turn very hard and a pretty red-orange color. The New Zealand Maori call it "Toa" or "warrior" because they make their fighting staves from it.

Here are some tool handles I made on Majuro Atoll. The axe handle and maul handles are ironwood.
I didn't make the small adze handle, but both adzes are cutoff pick axes. The rightmost maul head is breadfruit wood, lightweight so the neighborhood kids could help me split logs. The last photo is my pile of ironwood splitting wedges. Some of the wedges fell in the lagoon and when I found them again they'd turned into super-tough orange miracle material.

Step 2: Glue Up the Handle Blank

I didn't happen to have any hardwood branches or boards thick enough for a this pick handle, so I glued two free ash boards together with yellow glue. I used all my clamps as always.
If you don't have clamps or run out of them, homemade clamps, temporary nails and screws, innertubes, sandbags, innertube lashings etc. also work well.

This handle didn't stay glued very well after I left it out in the rain. So I used epoxy for the next one. I got the epoxy for free from a construction company who were getting rid of extra liquids due to flammable liquid storage rules.

Don't buy anything! Every person and company in the "developed world" has too much stuff
they need to get rid of.

Step 3: Draw the Handle on the Blank

Draw the side view of the handle on the board and saw it out. I used this nice bandsaw. Use whatever saw you have.
Trace the tapered socket of the pick head to get the dimensions of the top of the handle.
The big end of the socket sets the size of the top of the handle.
The little end of the socket sets the size of the rest of the handle. The handle has to pass through that hole in the pick head when you install it.
The handles I made are 36" long overall, make yours to suit yourself.

Step 4: Trace the Socket on the End of the Handle

The whole handle has to pass through the small end of the socket on the pick.
Draw that on the base of the pick handle.
The big end of the handle needs to mate with the big end of the socket.
Draw that on the top of the pick handle.

Step 5: Rough Shaping

This is a 24 grit carbide-chip metal disk from Porter-Cable. A real bargain at $7.
Watch out, it cuts human flesh extremely fast also. Wear a dust mask and safety goggles.

It doesn't matter what tools you use to carve the handle.
Use your favorite carving tools. A knife, planer, rasp, crooked knife, machete, axe, adze, etc. are all fine.

Step 6: Part Way There

So far so good, the head is sliding over the handle.
If you carve the handle small enough to be comfortable, it'll be plenty small to slide through the head.
Leave the big end big for now.

Step 7: Fitting the Head

When the head gets to the big end, bang the handle on a board as shown to seat it tight.
Then turn it over and bang the base on the board to remove it. Don't let the head pinch you when it falls.
Then look for rust marks to find the high spots.

Step 8: Rust Marks the High Spots

Where the handle touches the inside of the socket, there will be rust marks.
Carve them off.
Fit the head again.
Remove it again.
Carve off the rust marks again.
Repeat until the head is well fitted to the handle.

Step 9: The Head Is Fitted!

If your head is too new to be rusty, you can rub the inside with a pencil, crayon, dirt, etc.
Leave some handle above the top of the head, you don't want the head flying off.

Step 10: Final Sanding

Then I used a flap sander to sand off the marks from the 24 grit, and hand sanded after that.
The handle is done!
Some like their tool handles unfinished. The old wisdom is that it causes fewer blisters.
Picks tend to get left out in the rain, so I'll finish mine with linseed oil.

Enjoy your digging!