Food preparation for a ranger in the wild is not all that different from food preparation in this modern day and age, but there are some limitations.
Food has been and always will be heated over a source of fire, with a few modern exceptions. In Middle Earth, this is no different. Fire starting is at the head of every hot meal, though not every meal need be hot.
The traveling ranger should always be well-supplied with food that doesn't need to be cooked. For what their job required of them (constant vigilance while guarding an unaware populace), no ranger could ever be guranteed a hot meal on any given night. Thankfully, their close proximity to the shire allows us to take some hobbit-influence on food, which helps us dine rather well.
Hobbits were quite fond of apples, which are a readily available source of nutrition requiring zero prep time. Additionally, Bilbo's pantries were stocked full of pastries and other simple pleasures, so dry goods such as breads and cheeses are easy fare to carry in a bag and snack on should you be denied a hot meal at any time.
If you are blessed enough to be able to have a hot meal, the tools required are simple.
Most present-day rangers carry a pot known as a corn boiler, or simply a "boiler". These vessels can easily cook enough for a single meal, and are easy to carry in one's ruck or snapsack. The addition of a wooden spoon and a cup or bowl makes you plenty prepared for a wide variety of simple meals to be cooked over a fire.
The addition of a small frying pan may be desirable, as Sam was specifically quoted as carrying two shallow pans with folding handles that nested inside one another. This can add a bit to the variety of food you're able to process, but be warned. Frying food tends to be a noisy process, proven to attract orcs.
Tolkien's greatest treasury of food options comes in the very first chapter of The Hobbit, as a highly detailed list of Bilbo's pantry is unfolded, but there are other references to important diet staples found throughout his works that are easy to carry unrefrigerated in the woods. Some of the most important/useful for our purposes include:
- Salted pork
- Dried Fruit