This setting is meant for one-shots not for long campaigns.
XERC IS GREAT DUNGEON MASTER! SOME RATS TOLD ME THEIR STORY OF A MIGHTY RAID OF A GREAT VAULT KNOWN AS THE PANTRY GUARDED BY A FIERCE MONSTER! I SHALL RETELL THEIR TALE USING A RITUAL I LEARNED FROM SOME GOLIATHS AND A TINY, TINY HALF-MAN! IT IS CALLED DUNGEONS AND DRAGONS AND INVOLVES THROWING ROCKS UNTIL THINGS DIE! YOU WILL TAKE THE PLACE OF MY RAT FRIENDS AND LIVE THROUGH THEIR ADVENTURE!
Be aware of the rules.
Races. I suggest using the Ratfolk race in my 5etools module (also below). If you don't like the statblocks or wish to play another type of creature, such as a squirrel or hedgehog, you may use a different statblock, such as the Custom Lineage race from TCE.
Languages. The available languages are Beast Speech (equivalent to common), Rodent, Arthropod, Aquatic, Canine, Feline, Reptile, Avian, Infernal and Celestial. Languages specific to certain features such as Druidic and Thieves' Cant are also available. Most beasts cannot read or write, nearly all beast languages are spoken only. In some cases exceptions can be made or flavoured, such as a spellbook composed of smells or images rather than words or glyphs.
Distances and Speed. All rules regarding distances, are scaled down by a factor of 12. So a spell with a range of 120ft, now has a range of 120 inches and a table that normally takes up 2 5ft map tiles will now take up 288 5in tiles. This includes speeds, it may not be realistic to the actual speeds of the creatures, but it's easier for balance’s sake to scale everything equally.
Size. Creature sizes are changed to be relative to rats rather than humanoids. For example, a rat(folk) is medium and a small cat is large.
Weight. Carry limits and most item weights are scaled down by a factor of 32. So 1 pound equals 1/2 ounce. For example, with a strength score of 10, a human could carry 150 pounds, a rat could carry 75 ounces (or 4.7 pounds). Most items that you will use are scaled down versions. For example, a rat sized rapier (perhaps fashioned from a sewing needle) would weigh 1 ounce.
Religion. Patrons and Deities are flexible, but a fun idea may be to have cats, dogs and humanoids as gods and patrons. Obviously cats are evil/fiend and dogs are good/celestial.
Familiars, Pets and Summons. Any features or spells involving beasts, such as wild shape or familiars, should be discussed with DM to flavour it appropriately. For example, using insects rather than wolves and birds.
Money. Money is unlikely to have any significance beyond buying starting equipment (including spell components). You will almost certainly be unable to buy anything during the game.
Terrain. Terrains will almost exclusively be urban (Coastal and plains may occur in sewers and parks but don't count on it). Urban can be chosen for a ranger's favoured terrain. If you wish to be an "Urban Land Druid", I'd recommend the mountain or underdark spells.