You just need a thicknessing jig Jim, I just use a couple of blocks of wood, a clamp & a plane blade, there are plenty of examples on the Internet.
This one was the first to pop up on a Google image search which should give you a good idea.
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Just cut your veneer slightly over sized & keep moving the blade in a little every time you've pulled the veneer through until it's the thickness you want.
Hope tha helps.