So, I have a player who has a ranger with a previous backstory of being a self-sufficient hunter of rather large wild beasts. The player's style is heavily stealth based and almost exclusively ranged. He's mentioned that he'd like to multiclass to rogue to get the benefit of sneak attacks and other rogue-ish things. I'm fine with this in game, but I was wondering what's the best way to help him multiclass?

I know rangers are slightly underpowered at the best of times, and I've never really had a player who's multiclassed and been more powerful because of it (although maybe I've just had players who make silly choices, I don't honestly think it's my place as a DM to convince them out of character choices). Currently, I believe his plan is to get to Lvl 5 ranger (he's already lvl3) and then take a few rogue levels - I've recommended 3 rogue levels, but that can change - before going back to ranger until he gets bored with ranger benefits. 

I'm not too sure what this looks like in a character long term, or what I should get him to expect from the combo overall. What choices would benefit him the most in fulfilling his sneak-ranger idea? 

Important information about the character;  Str 13 / Dex 16 / Con 14 / Int 16 / Wis 13 / Cha 13 - Human variant. Ask if you want to know anything else. 

As you said sneak attack and assassinate can be good for a 3 level dip, (depending on how often they have the opportunity to surprise enemies) and they have the right idea multiclassing, the PHB ranger although rather lackluster is very front loaded, I don't see much if any benefit of going beyond level 12-15 (get him to have a look at the benefits and decide a happy medium) another one is to dip into fighter for the action surge and depending on the rogue dip and ranger levels go to 3-4 there for arcane archer or scout (if you allow UA content) I would also recommend based on those stats he should take the skulker feat or use ASI to boost his wisdom, if he is stealth-ing he will probably want pretty high perception/passive perception.

I’ve gone Lvl 9 GS Ranger before switching to Rogue for 2 levels. Expertise and Cunning Action are great plus Sneak Attack although tiny at 1d6 is still nice. 

Ranger (Gloomstalker)/ Rogue (Swashbuckler)

I am also thinking about building Ranger/Rogue Multiclass. It will be Inquisitive - hunter wood elf. I need help with suggestions

stats (after race bonus before any improvement): str 13, dex 18, con 14, int 12, wis 16, cha 11. Expertise skill will be stealth and Insight after muti-class. Others normal skills are: 
Acrobatics, Investigation, Nature, Perception, Survival

15 / 5 split:

1. have Vanish, but it overlaps with Cunning Action. The only benefit of Vanish is untrackable by nonmagical means.
2. 9 spell and 2 lv4 slot
3. sneak attack damage: 3d6 (avg 10.5)
4. 3 favoured enemies choose

13/7 split:

1. 2 more Expertise skill. Maybe pick Investigation and Perception
2. 8 spell and 1 lv4 slot
3. sneak attack damage: 4d6 (avg 14)
4. 2 favoured enemies choose

both of them will have both Uncanny Dodge and Evasion.

I am not sure which way I should choose. Please give me suggestions.

@Frozen_Fruit: How do you get Evasion with 15/5? The only things you get from going to 15 is another 4th level spellslot. the other things are redundant:
Vanish: Redundant with cunning action
Favoured Enemy: Do you really need advantage when tracking a specific monster type if you can take expertise in survival? The added language could be nice, but it's highly situational.
Archetype Feature: Gloomstalker Shadowy Dodge/Horizon Walker are redundant with Uncanny dodge.

In my opinion good points for Multiclassing are after
5 Extra Attack
9 Horizon walker Haste
11 Hunter/Gloom stalker/Horizon walker Archetype feature
13 Guardian of Nature 

Quote from Harachteh >>

@Frozen_Fruit: How do you get Evasion with 15/5? The only things you get from going to 15 is another 4th level spellslot. the other things are redundant:
Vanish: Redundant with cunning action
Favoured Enemy: Do you really need advantage when tracking a specific monster type if you can take expertise in survival? The added language could be nice, but it's highly situational.
Archetype Feature: Gloomstalker Shadowy Dodge/Horizon Walker are redundant with Uncanny dodge.

In my opinion good points for Multiclassing are after
5 Extra Attack
9 Horizon walker Haste
11 Hunter/Gloom stalker/Horizon walker Archetype feature
13 Guardian of Nature 

 Ranger (Hunter Archetype) can learn Evasion on lv 15 as one of the 3 choices. 

You are right about redundant. 

Expertise in survival (+4 at min) on all monster types is redundant with +5 a specific monster type. I didn't think about it.

The only questions are:

• Do I need advantage on Intelligence checks to recall specific monster type information? Answer: maybe using the researching features more often then.
• Do I want a two lv4 slot? and my personal answer is "no"

So I think I am going for a 13/7 split. Thanks for the help.

I'm currently running a Ranger/Rogue character. He's a Shadar-kai, 5th level ranger, 1st level rogue.

My current plans are to go to 7th level rogue, choosing assassin as the subclass, then switch back to ranger. He's a gloom stalker, and I've taken the Alert feat. This all combines to deadly effect in the right circumstances:

 Invisible in the dark to creatures with darkvision (thus gaining advantage on attacks); combined with pass without trace (ranger spell) makes him nearly impossible to detect

• Extra attack action first round of combat, which does additional damage if successful
• Ranger bonus action spell options (zephyr strike, hail of thorns) add to potential damage totals
• Sneak attack damage potential for every strike, depending on situation (see being invisible in the dark)
• At 3rd level of rogue (assassin), advantage on attack rolls against creatures that haven't gone in combat yet and hits made to surprised creatures are critical hits (see again being invisible and pass without trace)

And that's not even considering the reactive advantages of being a rogue (cunning action, evasion, uncanny dodge).

First strike capabilities are effing awesome with this multiclassing option.