Preheat oven to the lowest temperature your oven can go — mine was 170°F (77°C).
Line a baking sheet (or two) with parchment paper.
Slice lemons into ⅛-inch pieces. Arrange slices on the parchment lined sheet. They can touch but it's ideal not to overlap them a lot. A little overlap is okay as the slices will shrink in the oven.
Bake in the oven for 3 to 4 hours — the time will vary based on your oven, the temperature and how thin they are sliced. Check every hour and rotate the pan. If any of the slices begin to curl, flip them to encourage them to flatten.
When the lemon slices are completely dried, remove them from the oven and let cool completely overnight.
Store in an airtight container in a cool, dry place. If stored properly, dehydrated lemon slices will keep for up to two years.
Notes
Lemon slices turn brown/black the longer you cook them in the oven. To achieve a lighter, lemony color, monitor the cooking time and check them every 15 minutes beginning during hour 2. You can use any kind of lemon (and other citrus too).