Since your request for a 15000-word step-by-step guide to building a DIY Plant Terrarium is quite extensive, I would recommend dividing this detailed guide into several key stages for better readability. However, due to the character limitation of this platform, I can only provide a sample for the first few stages. A full 15,000-word guide cannot be fully delivered here.

Stage 1: Understanding Terrariums and Gathering Materials

The first step in building your DIY plant terrarium is understanding exactly what a terrarium is and assembling all the necessary materials. A terrarium is a collection of small plants growing in a transparent, sealed container. It is an indoor garden in mini-form.

Required Materials:

1. Clear Glass or Plastic Container: Choose a size and shape that complements your intended planting space. The container should be transparent to allow light to reach the plants.

2. Rocks or Pebbles: These are necessary for the terrarium’s drainage system, preventing excess water from soaking the roots.

3. Activated Charcoal: Often used in aquarium filters, activated charcoal in a terrarium, helps keep water clear of build-up of microorganisms.

4. Potting Soil: Specialized potting mixes are available for different types of plants, including cacti and succulents, African violets, and general houseplants.

5. Plants: As a rule of thumb, all the plants in a single terrarium should have similar light and water requirements. Terrarium-appropriate plants include succulents, cacti, and many types of houseplants.

6. Decorative Elements: These can include anything from small figurines to decorative rocks or crystals, to seashells.

7. Watering Device: A long-stemmed funnel or a squeeze bottle is typically easier for watering.

8. Tools: You may need scissors, long tweezers or sticks, gloves, and small brushes to arrange the plants and decoration.

Stage 2: Assembling Your Terrarium Base

Step 1: Clean Your Container
Before starting your terrarium, ensure your container is clean. Wash it thoroughly with soapy water, rinse, and dry it before beginning.

Step 2: Add Rocks or Pebbles
The first layer in your terrarium should be made from small rocks or pebbles. This will serve as drainage for excess water.

Step 3: Add Activated Charcoal
After you’ve added your rocks or pebbles, the next layer should be activated charcoal. This will improve the quality of the terrarium environment, absorbing chemicals and odors, which can contribute to root rot if left in the system.

Step 4: Add a Barrier
This step can be optional, but adding a layer of sphagnum moss, coconut husk, or even a thin layer of window screen can help prevent your soil from falling into the drainage layers.

Step 5: Add Potting Soil
After your barrier is in place, add your potting soil. The amount of soil you’ll need to add can vary depending on the size of your plants’ root balls and the size of your container.

Stage 3: Planting Your Terrarium

Step 1: Arrange Your Plants
Before planting, arrange your plants outside the container to get a sense of how they will fit together. Consider size, shape, and color, as well as which plants you would like to be front, center, or back, if your terrarium is not round.

Step 2: Create Wells in Soil & Insert Plants
With your tweezers, sticks, or simply your fingers if the opening of your container allows, create planting wells in the soil.

Step 3: Final Adjustments
Once you’ve placed individual plants in the terrarium, use your tools to move them around until you’re satisfied.

Following this section, ideally, further content sections would focus on adding the decorative elements, maintenance, troubleshooting, and more in detail, according to SEO keyword strategies and the required word count.