The combo is:
Dro-m’Athra Reaper + Namira’s Ring + Journey to Sovenguarde
In that order.
The Reaper gives you health when creatures leave discard, which triggers the Ring. By playing Journey you deal massive damage in one turn. It is easier to reach the threshold to kill your enemy instantly if you play 2 Reapers in one turn.

Magicka needed for 1 Reaper: 16 (4 ramps)
Magicka needed for 2 Reapers: 18 (6 ramps)
The magicka required means you need to often use Odiniran Necromancer to bring back ramp like Tree Minder and Thorn Histmage. Keep this in mind, but always put survival first against aggro or mid.

Galyn is used to target either Necromancer or Reaper. Reaper lets you find more reapers for combo, and use the ones in your hands to gain health and trade. Necro lets you ramp more, lethal remove creatures, and most importantly, lets you bring back Galyn to shuffle more copies. Generally, it’s better to target Necromancers, but if you want to close a game out against control targeting the Reaper might be better.

Remember that if you are losing and need to stabilize playing 2 of your Reapers is safe because you can shuffle more back in.

If you think you are fighting Aggro, it is usually worth it to hard mulligan for Necromancers Amulet for lifegain. You will almost certainly draw two drops, but there is only 3 amulets in the deck.

Good luck, as long control doesn’t use memory wraith you should win automatically.