Some people are going to look at this, and say Boots of Celerity? Really? Scrub... but no, hear me out, a fast Ymir, is a scary Ymir, get up in their faces and freeze, catch up and easily wall in front of them, or my favorite, Pop heavenly agility, zoom past with speed buff and cast your ult behind the enemy team, they have to go through it, waste skills to get past, or go through your team, your almost guaranteed to have someone on the enemy team killed.
I start Level 1 Boots of Celerity and Hand of The Gods, This will help you get the green buff at the start to have more sustain.
On my first back Ill grab Heavenly Agility or Creeping Curse, whichever you'd prefer.
Heavenly Agility is great to keep up with fleeing enemies, Ganking from the jungle, cathcing people off guard with a quick run up freeze, and getting your ult off behind the enemy team.
Creeping Curse helps with getting your ult off on as many people as possible, Pop it, Ult, unless they have a leap, your going to hurt, a lot.
Ill get level 2 boots and start either Hide of the Nemean Lion or Stone of Gaia next, depending on the lane setup, if its a mix of Magical and Physical, figure out which will do more damage and get protection for that one, if its still equal/you don't know, I usually grab Stone of Gaia for the Sustain alone.
As soon as you finish Hide/Stone, build the one you didn't grab, then finish your boots. Pick up Beads or Aegis, depending on their CC/Lack of CC.
Once you get your level 2 boots and either defense item, you can start ganking, grab that speed buff and go mid, communicate with your team and let the enemy mid push out, come from behind, freeze and mid should die or at the very least need to go back
At this point It should be Mid game and you should be team fighting, so grabbing Void Stone is great here, gives your magic resist, Magic Pen, helps your Ult, helps your Magical based teammates, combined with your Passive, you should start raking in the assists/kills.
Grabbing a Mystical Mark gives you a little bit of extra damage output while ulting, as well as that Physical Protection.
You can reverse the build order here depending on if you need the Physical or Magical Defense, you should be tanky enough by now to get either one, but if you notice more damage output from one or the other, you should take the one you need
From here on out, its your call honestly, the game shouldn't be going much longer than this, you could grab
-Hide of Leviathan for some more protection
-Midguardian Mail for that auto attack slow
-Breastplate of Valor for that CDR and more Physical Protection
-Gem of Isolation for HP/Mana/Ability Slows
-Silverfox Girdle for HP and more sustain
-Sovereignty if they have a lot of Physical and your a team-bro
-Fatalis is funny for move speed + attack speed