After casting Old Stickfingers for X=3 you can usally win on the next turn:
The first step is always to cast a reanimate spell on Young Necromancer. From there on you can go different directions.

COMBO 1 with 1 black mana up , 1 addition creature on the battlefield (usually Old Stickfingers) , 2 cards (or 3 if creature ist a token) in grave after casting reanimate spell:

The first description takes track of life loss, needed cards in graveyard and manapool before each step with following short terms:
- LP... total life lost
- Gr... total change of (irelevant) cards in graveyard   

- {b}, {g}, {b/g} ... black or green mana

Combo steps - searched cards get usually played and used directly, sac outlet is always Razaketh:

1. (-0 LP, -0 Gr, 1x{b}) reanimate Young Necromancer --> return Razaketh, the Foulblooded
2. (-0 LP, -2 Gr, 1x{b}) sac other creature for Feign Death, play it on Young Necromancer (very important: let your commander stay in your graveyard to be able to exile it)
3. (-2 LP, 0 Gr, 0x{b}) sac Young Necromancer for Mox Amber--> Young Necromancer revives --> return Nyxbloom Ancient
4. (-4 LP, -2 Gr,3x{b}) sac Young Necromancer for Cultivator's Caravan to get green mana
5. (-6 LP, -2 Gr, 3x{g}) crew and sac Cultivator's Caravan for Harrow to get green and black mana
6. (-8 LP, 0 Gr, 3x{g} 3x{b}) sac Nyxbloom Ancient for Diregraf Rebirth --> return Young Necromancer --> return Nyxbloom Ancient
7. (-10 LP, -2 Gr, 2x{g} 2x{b}) sac Young Necromancer for The Great Henge (note: -12 LP during step)
8. (-10 LP, -2 Gr, 6x{g} 2x{b}) sac Nyxbloom Ancient for Paradox Engine 
9. (-12 LP, -2 Gr, 1x{g} 2x{b}) cast Diregraf Rebirth on Nyxbloom (Henge and Mox get untapped for mana and and life)
10. (-10 LP, -6 Gr, 4x{b}) cast Old Stickfiners from command zone for a up to 10 mana then sac it for Verdant Command

Due to Paradox Engine,  The Great Henge and Mox Amber untap after each spell. Therefore, you don't loose more than 12 life (of course even less if you manage to have some combo cards already in hand) and get as much mana as needed with Nyxbloom Ancient on battlefield.
Lastly search for Pestilent Cauldron, you can now mill out the opponent with it by casting some spells (like your commander) to untap it multiple times because of the lifegain provided by The Great Henge each time you tap it (you can even exile cards like Gaea's Blessing on the stack with an instant in hand). 
That way you only need more than 12 life at the combo turn.

COMBO 2: The combo is a lot easier if you have 2 additional mana up since you can directly search for The Great Henge. This way ist actually the most common one and you only have to spent 5 life:
1. same as in combo 1
2. same as in combo 1
3. sac Young Necromancer (which brings it and Nyxbloom Ancient back) for The Great Henge
4. sac Young Necromancer for Verdant Command
5. sac tokens for Paradox Eengine and Mox Amber
6. cast Old Stickfingers from command zone
7. sac it to search for Pestilent Cauldron and cast enough follow-up spells for the win

Combo 3: If you have 2 additional cards in grave for the combo 1, there is a way which causes you to loose only 9 life:
1. start same as in combo 1
2. sac Young Necromancer for Mox Amber --> Young Necromancer revives --> sac it with its ability on stack for Cultivator's Caravan --> return Young Necromancer --> return Nyxbloom Ancient
3. crew and sac Cultivator's Caravan for The Great Henge
4. same as in combo 2
5. same as in combo 2
6. same as in combo 2
7. same as in combo 2

It is even possible to win of more than 4 life in combo 2 respectively 8 life in combo 3, if you can cast a spell with 2x{g} after searching directly for Paradox in step 4.

Some other helpful notes:
- If you have Feign Death or Malakir Rebirth in your hand, you are not required to have a creature on the battlefield since you do not have to search for it. Additionally don't forget potential land creatures like Blinkmoth Nexus and Mutavault.
- Against counterspells you can hold a discard spell for your comboturn to get through one possible counterspell or just search for Veil of Summer or Defense Grid.
- If you can't win right away (for example if opponent represents a removel and you do not have additional mana and creatures to protect the combo), you can just reanimate Young Necromancer and Razaketh, the Foulblooded and leave 1x{g} open. Therefore, you are able to protect Razaketh, the Foulblooded by searching for Tamiyo's Safekeeping or Veil of Summer and/ or have a set up to win next turn by searching for another reanimate spell.
- If you have Nyxbloom Ancient or Razaketh, the Foulblooded in your hand, it is seginificantly harder to win because besides having to discard or hardcast them to win, Old Stickfingers doesn't stay on the battlfield as a creature to sacrifice if you reanimate the other two creatures. In case of Nyxbloom Ancient you can atleast search for Bone Shards or sometimes Pestilent Cauldron with Razaketh, the Foulblooded on battlefield which lets you discard and in case of Pestilent Cauldron produce more tokens to sac. In the other case you usually try to combo on 8 mana by hardcasting Razaketh since you will have 9 mana after bringing Nyxbloom Ancient to the battlefield with Young Necromancer.
- Against the biggest weakness graveyard hate: If you could not make the way free with discardspells, you can play around some hate cards if you aren't under a lot of pressure. Boseiju, Who Endures lets you kill disrupting artifacts and enchantments. Sorcery speed interaction is beatable by either having Nissa of Shadowed Boughs or Liliana, Death's Majesty with enough loyalty in play before casting Old Stickfingers since then you can combo off in the same turn.