Throne of Eldraine has given lots of new toys for control decks. Namely, Drown in the Loch, Into the Story, Murderous Rider, Vantress Gargoyle. This deck uses mill to utilize those new powerful cards. Decking your opponent is a viable win condition in a long games, but it is not a mill deck, it is a classic draw-go control deck.

About card selection:

Murderous Rider, Disfigure and Drown in the Loch are quite obvious.

Vantress Gargoyle - this card doesn't need much to turn into Tarmogoyf with wings. Remember that some green decks side in Lovestruck Beast to fight fast agro? Now this deck has 4 of similar roadblocks maindeck for a cheaper price. Gargoyle holds off aggression during early game and beats opponent fast later. Treat it as removal most of times: 1U for killing a Questing Beast is a nice price to pay. Keeping 4 cards in hand can be tricky - sometimes it means skipping a landdrop. But usually you need gargoyle on guard only for first 2-3 turns. Natural enemy of Vantress Gargoyle is Wicked Wolf, so watch out for those and try to intercept them with counterspells.

Overwhelmed Apprentice is a small buddy who fills few roles: he serves as an Opt digging you for the certain cards; he fills opponent's graveyard to activate your Drown in the Loch and he slows down fast agro. Merfolk Secretkeeper and Wall of Lost Thoughts are just bad. If 60 cards is like 20 life, then milling 4 cards is like pinging for 1. Do you really want to play 0/4 deal 1 damage to opponent for 2 mana in your deck? Merfolk does nothing expect for mill 4, and this deck should be able to activate Drown in the Loch and Into the Story without sacrificing a whole card to it.

Best way to do this are Didn't Say Please and Thought Collapse. These counterspells put 4 cards in a graveyard without wasting a card and still relevant on any stage of the game. Maybe later this deck will move to 5+ such counterspells, but currently meta is aggressive, so I prefer having full set of Disfigure.

Into the Story is one of the best drawing engines in years. It is even better than Fact of Fiction. But with only 4 copies of it as card drawing engine you often will run out of cards. That's why there are additional Bond of Insight. In a deck with lots of instant and sorceries Bond of Insight is a more reliable Drawn from Dreams, which also contributes to milling the opponent. The fact that it returns spells from the graveyard instead of drawing them means that against Fires decks you have access to 14 counterspells, against temur Reclamation you have 8 discard spells maindeck, against gruuls - 8 Enter the God-Eternals.

Enter the God-Eternals suprisingly works fine as the main wincon. Especially given the fact you can return it with Bond of Insight. Add one from the sideboard vs decks with Questing Beast.

Noxious Grasp is still in the maindeck because it hits two major targets - Teferi, Time Raveler and Domri, Anarch of Bolas who otherwise will blank your counterspells. Having just 4 Murderous Rider is not enough for negating same number of threats because that means in 50% of cases your opponent will draw more Teferis than you had drawn Riders. Plus Noxious Grasp can be reused with Bond of Insight.

In the sideboard Cry of the Carnarium comes vs mardu knights and adventure decks; Thought Distortion and Narset, Parter of Veils are cards vs other control decks like UW or temur Rec and Esper Stax, temur reclamation is a hard matchup; Noxious Grasp is for bant ramp, Epic Downfall for Cavaliers.

 Thought Erasure here is worse than in esper control because of lack of Teferi, Time Raveler. But you still need it vs Hydroid Krasis.

1BB and 1UU demand 18 colored sources = 12 'duals' + 6 basics of each color. That's why deck has to play both Dismal Backwater and Dimir Guildgate.