Answer: not hard, just tedious. But totally worth it! Here's what I did.
Step 1: Google "creating a hat block".
That turned up this video, which may be slightly goofy and not exactly what I wanted, but it did give me the idea to use expanding insulation foam to create a block from an existing hat.
Step 2: Figure out a way to not destroy the existing hat in the process of making a mold.
The video said to use cooking spray to grease the inside of the mold hat so the foam wouldn't stick to it. I'd like to still be able to wear that hat after this, so I experimented with a number of different materials available at your local grocery store. I eventually settled on using parchment paper to line the hat crown with, since my hat had a square crown and no fancy curves to it.
Parchment paper mold.
Inside the mold form.
Step 3: Apply expanding foam.
Hey, guys? When they say the foam will expand, they mean IT. WILL. EXPAND. In case you were wondering.
Follow directions better than I did...
Step 4: Poke holes in too-big expanding foam so the inside will cure.
Obviously, you don't have to do this if you didn't overdo the foam in the first place.
Yeah. Overkill, much?
Step 5: Pry giant lump of foam out of hat mold.
While I don't recommend over-filling the mold to the extent that I did, I would recommend giving yourself a little bit of extra so you can trim the bottom off level, like you would with a cake you're going to decorate. This is less tasty. I ended up trimming mine off to size, rather than scoring a groove in the bottom like the video recommends. I used a giant bread knife, which worked well, and I didn't even cut myself, which was even better.
Step 6: Procure unsized, unblocked hat blank of your choosing.
I ordered from Hats by Leko, and was extremely pleased. The $28 minimum order is a little troublesome, but when you factor in other supplies (I got millinery wire, velvet leaves, hat sizing, and two hat blanks), it's not so bad. Their shipping is what I would consider steep, but USPS Priority ain't free, and it got to me quite quickly.
Soft, unblocked straw capelines.
Step 7: Block hat.
This will be messy. Wet the straw, start stretching, pulling, poking, prodding, etc. Your straw hat blank will probably shed little bits all over the place.
With your block being foam, you can stick pins into it wherever you need to.
At this point I realized that my hat blank wasn't going to play nice in the transition from crown to brim, so I ended up taking a tuck in the straw to get rid of some excess material. It worked, and the seam will be covered with a band and flowers, so no harm done. I then turned the raw outer edge of the brim, pinned, and stitched it. For all the stitching, I just used my machine, and kept the straw just a bit damp so it wouldn't be brittle.
Lovely, pliable damp straw. Clammy.
Step 8: Size hat.
"Sizing" in this case refers to adding a stiffening agent to your hat. This keeps it from drooping out of shape in a light rainstorm...although I'd try to avoid rainstorms (light or otherwise) either way. I used the Hydrolac B-5 powder from Hats by Leko, and while I was very dubious about it initially, I was pleasantly surprised. It starts out looking like cake batter and transforms into a clear, amber-colored liquid over the course of about an hour.
Starts out goopy...
...Turns clear. Weird. But effective!
I did not use a spray bottle, which is what the directions said to do. I was doing this in my living room with silk and organdy laid out all over the place and that seemed like asking for trouble. I used a junky paint brush instead, and it worked just fine.
Step 9: Try hat on, realize it's too small.
My theory is that the foam shrank a little after I had un-molded it and it had cured a little farther. This is no doubt due to my ineptitude with the foam in the first place. Either way, I fixed this by wrapping a strip of fun foam--you know, that spongy craft stuff you cut little shapes out of--around the block and re-wetting the hat, re-stretching, and re-sizing it. It took me maybe half an hour. It was still irritating.
Incidentally, you could save yourself the trouble my measuring first, and I do highly recommend fun foam for padding if you end up needing to do so.
Step 10: With the hat now fitting properly, stitch millinery wire to inside of brim.
What it says on the tin--a single round of millinery wire will help the brim keep it's shape once you've steamed it in place.
I used a conventional iron and liberal application of the "steam" button for this, and just prodded the straw into its final shape while it was still slightly pliable. The Hydrolac B-5 directions say that steaming will set the sizing, too. This is also when I added the curves to the brim, steaming them in place as I went.
Next, we'll talk about how to finish the hat...but that's for later, since I still have 3 dresses for me and a dress for my mom to finish. Need to get a move on!