I don't see where you say what Kindle model this is intended for. But recent Kindles are likely to require 1.8v. Some people have said that 1.8v models are tolerant of using 3.3v UARTs (serial port adapters). But you risk hardware damage if you try that.
You could purchase or construct a
level shifter,
voltage divider, or even pair a zerner diode with a pull down resistor to translate 3.3v logic to 1.8v. But as you can buy proper
1.8v hardware for $2 - $10 shipped, wouldn't that be a better approach? $10 or less on Amazon. As low as $2 on Ebay or Alibada but quality control and shipping times are not good at that price point.
edit: Looking at your pictures, the relevant question seems to be how to use the pictured board? Didn't it come with any documentation? It appears to be a cheap Chinese buck converter. A search for "HW-613" would probably pull up a manual. But it's pretty straightforward all the same. You will need an input voltage that is at least ~2v DC higher than you expect out. It's not entirely clear what the max voltage rating here would be. I wouldn't trust it with more than 15v in.
VO+ voltage out, positive DC
GND ground
VI+ voltage in, positive DC
EN enable port
Adjust output voltage via the pot in the upper right hand corner. Or for a fixed voltage, cut the trace to the pot, labelled "adj" and bridge (solder) the two pads next to the silk-screened voltage desired.