In order to create the Bomb, you will need a Crimson Ore and two Sand-category ingredients. This is a very simple synthesis, with no other requirements.
Gathering the Ingredients
You can find all ingredients for this recipe at the path west of the Atelier, leading to the Flower Ring.
For the Crimson Ore, look for the rocks with red ores on them. Use the staff on these rocks to obtain the Crimson Ore.
You can also find two Sand-category ingredients here. Collect the small mounds of dirt in order to obtain Ashen Sand.
You can collect these small piles of rubble to obtain Burning Sand.
These two ingredients have different Element values. You will need both of them if you decide to create a stronger Bomb later.
Synthesizing the Bomb
After obtaining the ingredients, return to the Atelier and synthesize the Bomb.
Add a Crimson Ore to the Effect 2 loop in the center.
Use Ashen Sand in the Effect 4 loop on the bottom.
Insert Burning Sand in the Effect 3 loop on the top right.
With all core loops completed, you can synthesize the Bomb.
If you wish to create a stronger Bomb, prioritize leveling up the Effect 2 loops. These two loops are fairly easy to level up, and will add Fire Damage M as an effect.
Once you create the Bomb, head west of the Atelier and continue towards the Kark Isles. This will be a long journey, so make sure to upgrade your party’s equipment beforehand. Try synthesizing healing and offensive items as well, since you will run into stronger enemies.