Entering Arcane Trickster is very simple if you use Unseen Seer.
Rogue 1/Wizard 4/Unseen Seer 1 is all you need.
Caster level 5, enough for 3rd level spells. Character level 6, enough to have 9 ranks in skills.
2d6 sneak thanks to Damage Bonus adding 1d6 to the 1d6 you get from your rogue level.
My standard magical trickster looks something like this, it's a ranged attacker:
Human Rogue 1/Focused Conjurer 5/Unseen Seer 10/Arcane Trickster 4
Str:10
Dex:14
Con:12
Int:18
Wis:10
Cha:10
BAB: 12
Base saves: 5/10/15
Feats:
1: Able Learner, Darkstalker
3: Invisible Spell
6: Acidic Splatter, Extend Spell
8: Silent Spell
9: Practised Spellcaster
12: Metamagic School Focus [Conjuration]
15: Persistant Spell
18: Quicken Spell
Sneak Attack:
7 d6+8d6 for Persistant Hunter's Eye
CL:
21 Normal
22 for Acid Spells
24 for divinations
25 for divination acid spells (which there are none)
Schools Banned:
Evocation
Necromancy
Enchantment
Spells:
0:16
Resistance, Acid Splash, Caltrops, Detect Poison,Detect Magic, Read Magic, Ghost Sound, Silent Portal, Launch Item, Stick, Mage Hand, Mending, Message, Open/Close, Arcane Mark, Prestidigitation
1:9
Grease, Obscuring Mist,True Strike,Nerveskitter,Disguise Self,Silent Image, Color Spray, Feather Fall, Golem Strike
2:4
Glitterdust, Invisibility,Mirror Image,Cloud of Knives
3:4
Stinking Cloud,Fly,Displacement,Shrink Item
4:5
Greater Invisibility,Greater Mirror Image,Orb of Acid,Arcane Eye,Dimension Door
5:5
Wall of Stone,Teleport,Major Creation,Telekinesis,Prying Eyes
6:3
Acid Fog,Greater Dispel Magic,Mordenkainen's Lucubration
7:3
Greater Teleport,Limited Wish,Spell Turning
8:4
Mind Blank,Superior Invisibility,Discern Location,Moment of Prescience
9:6
Gate,Black Blade of Disaster,Foresight,Time Stop,Shapechange,Wish
Advaced Learning:
Hunter's Eye
Find the Gap
Gravestrike
It's pretty average, I'm sure it can be a lot better done by someone who's better at it than me, but the basic gist is there. Pick up Hunters Eye from the Ranger spell list through unseen seer, persist it. 15d6 sneak. Use a Persisted, Invisible Cloud of Daggers to get a free action attack each turn at 1d6+5 that gains sneak damage if you're invisible. Use Acidic Splatter to get a 30 feet orb of acid for 4d6 that also gets sneak attack when you're invisible, that's 19d6 acid damage as a ranged touch...
And these are your stand-by options when you're not committing spells for something important. If it's not enough trickery with Illusion spells, skipping being a focused conjurer and picking up Enchantment spells could be an option, if that's what you want. It has an easier time to get sneak attacks through than a regular rogue thanks to Gravestrike and Golem Strike (allowing to SA undead and constructs respectively), they happen to be Divination spells even.