Cast Marketback Walker by paying 1, causing it to enter the battlefield with a +1/+1 counter on it.
Activate Together Forever by paying 1, targeting Marketback Walker.
Activate Ashnod's Altar by sacrificing Marketback Walker, adding CC.
When Marketback Walker dies, it and Together Forever trigger.
Resolve the Marketback Walker trigger, causing you to draw a card.
Resolve the Together Forever trigger, returning Marketback Walker from your graveyard to your hand.
Repeat.
This clever loop transforms Marketback Walker into an endlessly recycling engine, fueled by the mana from Ashnod's Altar and protected by Together Forever's ability to bounce the automaton straight back to your hand upon its demise. Stenn, Paranoid Partisan sits quietly on the battlefield, and while its cost-reduction isn't strictly required to execute the baseline sacrifice loop, its presence helps grease the wheels of your broader strategy. Each iteration of this cycle costs just one mana to set up and two to sacrifice, netting you two colorless mana from the Altar, drawing a card for every counter that was on the Walker, and generating an endless stream of death and sacrifice triggers.