How do I make a "White Decal".
I need to make an Afrika Korp insignia, the white palm tree
with a white swaztika in the middle. 1/35 scale.

I have "Clear" and "White" decal film but I need a white design...

I think there's two reasonable ways to come up with one. (or three, if you're mad enough to cut out the insignia from solid white decal film... )
You could print the insignia to clear decal film, but it's not possible with regular printers. (white being made with the absence of color)
Special ALPS printers can print white, but from what I've heard, they're quite expensive.
Another option is to hunt around for aftermarket decals, or left-overs from some kit decals.

Btw, what scale are you looking for?
I remember dragon's DAK Panzer I has some of those insignias. I could send you some, but I'm not sure if there'll any left over from the scheme that has them.
I'll check it out.

As stated above, you can't easily or cheaply print white decals. Here are a couple options though.

You can print them on the white decal paper and color the area around the decal the same as your base color on the vehicle. It is called reverse printing.

Another option is to print it in a gray/tan color so it looks like a dirty white. This can be printed on the clear film. It has to be darker than the vehicle color though, or it will disappear into the background color.

The best method is to silk screen the image onto clear decal film using white pigment. It's not as expensive as you would think, especially if you know someone in the business. Prepainted model railroad rolling stock is generally done this way, just not onto clear film.

Apparently, the one scheme that has the DAK insignias uses two markings, the sheet has four (two styles are present, with a palm with long or short leaves), so I guess I have some spares.