I have been following this tutorial for my mod, though I am wondering how I add multiple items in the ModRecipeProvider class. 

        ShapelessRecipeBuilder.shapeless(RecipeCategory.MISC, ModItems.LEMON_JUICE.get(), 4)
                .requires(ModItems.LEMON.get())
                .save(pWriter);

I am wanting to add the Glass Bottle to the recipe though I am not sure how. (As seen in the .json file below).

{
  "category": "misc",
  "ingredients": [
    {
      "item": "droidsancientrelics:lemon"
    },
    {
      "item": "minecraft:glass_bottle"
    },
    {
      "item": "minecraft:glass_bottle"
    },
    {
      "item": "minecraft:glass_bottle"
    },
    {
      "item": "minecraft:glass_bottle"
    }
  ],
  "result": {
    "count": 4,
    "item": "droidsancientrelics:lemon_juice"
  },
  "type": "minecraft:crafting_shapeless"
}

Would this be easy to perform?

Just add another .requires after the first one. You can even specify how many should be needed. In your case, that would look like: 

ShapelessRecipeBuilder.shapeless(RecipeCategory.MISC, ModItems.LEMON_JUICE.get(), 4)
                .requires(ModItems.LEMON.get())
                .requires(Items.GLASS_BOTTLE, 4)
                .save(pWriter);

Oh wow, It was really that simple? Sometimes I get so confused...