After playing a few games, it was noticeable that what’s needed the most is what the item combinations are. You would need this every time you get an item until you memorize all combinations… or if Riot implements a feature that displays the combinations within the game that you can see all the time. Until then, you can use this for reference.

B.F. Sword:+20 Attack Damage
└ + B.F. Sword = Infinity Edge : Critical strikes deal +200% damage.
└ + Recurve Bow = Sword of the Divine : Each second, the wearer has a 5% chance to gain 100% critical strike.
└ + Needlessly Large Rod = Hextech Gunblade : Heal for 25% of all damage dealt.
└ + Tear of the Goddess = Spear of Shojin : After casting, wearer gains 20% of its max mana per attack.
└ + Chain Vest = Guardian Angel : Wearer revives with 300 health.
└ + Negatron Cloak = Bloodthirster : Attacks heal for 35% of damage.
└ + Giant's Belt = Zeke's Herald : Adjacent allies gain +12.5% attack speed.
└ + Spatula = Youmuu's Ghostblade : Wearer is also an Assassin.

Recurve Bow: +20% Attack Speed
└ + Recurve Bow = Rapid Firecannon : Doubles attack range, attacks can't miss.
└ + Needlessly Large Rod = Guinsoo's Rageblade : Attacks grant 5% attack speed. Stacks infinitely.
└ + Tear of the Goddess = Statikk Shiv : Every 3rd attack deals 100 splash magical damage.
└ + Chain Vest = Phantom Dancer : Dodges all critical strikes
└ + Negatron Cloak = Cursed Blade : Attacks have a chance to shrink (reduce enemy's star level by 1).
└ + Giant's Belt = Titanic Hydra = Attacks deal 10% of the wearer's maximum HP as bonus splash damage.
└ + Spatula = Blade of the Ruined King : Wearer is also a Blademaster.

Needlessly Large Rod: +20% Spell Damage
└ + Needlessly Large Rod = Rabadon's Deathcap : +50% ability power.
└ + Tear of the Goddess = Luden's Echo : Spells deal 100 splash damage on hit.
└ + Chain Vest = Locket of the Iron Solari : On start of combat, all adjacent allies gain a shield of 200.
└ + Negatron Cloak = Ionic Spark : Whenever an enemy casts a spell, they take 100 damage.
└ + Giant's Belt = Morellonomicon = Spells deal burn damage equal to 10% of the enemy's maximum health per second.
└ + Spatula = Yuumi : Wearer is also a Sorcerer

Tear of the Goddess: +20 Starting Mana
└ + Tear of the Goddess = Seraph's Embrace : Regain 20 mana each time a spell is cast.
└ + Chain Vest = Frozen Heart : Adjacent enemies lose 20% attack speed.
└ + Negatron Cloak = Hush : Attacks have a high chance to silence.
└ + Giant's Belt = Redemption : On death, heal all nearby allies for 1000 health.
└ + Spatula = Wearer is also a Demon.

Chain Vest: +20 Armor
└ + Chain Vest = Thornmail : Reflect 25% of damage taken from attacks.
└ + Negatron Cloak = Sword Breaker : Attacks have a chance to disarm.
└ + Giant's Belt = Warmog's Armor : Wearer regenerates 2.5% max health per second.
└ + Spatula =

Negatron Cloak: +20 Magic Resistance
└ + Negatron Cloak = Dragon's Claw : 83% magic resistance.
└ + Giant's Belt = Zephyr : On start of combat, banish an enemy for 5 seconds
└ + Spatula = Brawler's Gloves : Wearer is also a Brawler

Giant's Belt: +200 Health
└ + Giant's Belt = Warmog's Armor : Wearer regenerates 2.5% max health per second.
└ + Spatula = Wearer is also a Glacial.

Spatula:???
└ + Spatula = Gain +1 team size.