I am making a very specific Ranger/Rogue multiclass that heavily relies on getting sneak attacks. What are some reliable ways to gain advantage on attacks in combat, especially at early levels?
There are multiple ways
The simplest, and most reliable way to enable sneak attack is to have an ally that threatens your target from within 5 feet. Outside of that, there is a number of ways to reliably get advantage as a pure rogue as explored here, including, without feats:
Using your Cunning Action to hide when not in plain sight and attacking from being hidden (at second level)
the Steady Aim rogue feature from Tasha's Cauldron of Everything (an optional rule, at third level)
A trained Mastiff and Animal Handling to make it attack. As it only has a few hp, this approach will become impractical after the first two or three levels.
And needing a feat, which would require you to be a variant human to get access during the first 3 levels:
- getting a familiar, for example via the magic initiate feat, which can also give you yet another way via the true strike cantrip (you can also get them with the Arcane Trickster rogue subclass at third level, kudos to @draconis; true strike will cost your action to cast, so may not be that efficient)
- Using a horse and the Mounted Combatant feat (although that is an unlikely tactic in most dungeon environments)
- Using the Martial Adept feat for limited maneuvers per short rest, like Feint
Lastly, considering you are playing a ranger/rogue: one of the more effective ways if the rest of the party has darkvision is a Gloom Stalker ranger. It will give you access to Umbral Sight at third level to make you invisible in darkness — which will both give you advantage, and opponents disadvantage. At night or in dark dungeons this is a very reliable tactic, see here for an example of the results. It however will not work at the very start, as you need at least 3 levels of ranger and 1 of rogue.
Since we don't know about the rest of your party, the low-hanging fruit is the Rogue's own class features.
At second level, you gain the Cunning Action feature, which lets you Hide as a bonus action. Successfully hiding from a creature (which depends on the environment and your Stealth roll) will give you advantage on attack rolls against it.
At third level, if you're using the optional rules in Tasha's Cauldron of Everything, you gain the Steady Aim feature, which lets you sacrifice your movement to gain advantage on a single attack roll.
Groody mentions the find familiar and true strike spells, available to a variant human through Magic Initiate. But a non-human can also get those spells by taking the Arcane Trickster subclass at third level; while most of your spells are limited to the enchantment and illusion schools, your cantrips are not, and you're allowed to pick one levelled spell outside those schools at third level.
Since you do not specify subclasses or how many levels of each class you intend to take, I am not sure this suggestion can work for you, but I'll add it for completeness, as I do not see it mentioned in the other answers.
If you take 13 levels of rogue, and you go the arcane trickster route, you get at 3rd level:
Mage Hand Legerdemain
Starting at 3rd level, when you cast mage hand, you can make the spectral hand invisible, and you can perform the following additional tasks with it:
- You can stow one object the hand is holding in a container worn or carried by another creature.
- You can retrieve an object in a container worn or carried by another creature.
- You can use thieves’ tools to pick locks and disarm traps at range.
You can perform one of these tasks without being noticed by a creature if you succeed on a Dexterity (Sleight of Hand) check contested by the creature’s Wisdom (Perception) check.
In addition, you can use the bonus action granted by your Cunning Action to control the hand.
and at the 13th it gets augmented (emphasis mine):
At 13th level, you gain the ability to distract targets with your mage hand. As a bonus action on your turn, you can designate a creature within 5 feet of the spectral hand created by the spell. Doing so gives you advantage on attack rolls against that creature until the end of the turn.
This ties up your bonus action every turn (and 13 levels of arcane trickster), but it guarantees advantage.