Flattening the bottom

A friendly 3D print needs to have a solid, flat base. There is more than one way the bottom of a model can be flattened. For one, the geometry can be edited to be flat. Alternatively, a floor object can be created and cut out of the object using a Boolean modifier. Since this project is using the subdivision surface modifier, editing the geometry can be complicated. So, the floor method will be used in this project.

Looking at the model from the front orthographic view (NumPad 1 and NumPad 5), it's clear that the parts of the tentacles that sit below the x and y axes (the red or green line depending on the view) is where the model needs to be cut off in order to create a flat, printable base:

So, let's get started:

1. First, exit the Edit Mode (Tab).
2. Ensure that the 3D cursor is at the 3D View origin point (Shift + C). Then, create a cube (Shift + A) by navigating to Mesh | Cube:
   
3. The cube cannot be seen in the solid view because it's completely inside the octopus, so with the cube still selected, switch to the local view (NumPad /) to view the cube by itself:
   

   The cube is half above and half below the origin. Fortunately, there's an easy way to create a floor and ensure that no matter how it's scaled, the top remains on the XY plane.

4. In the Edit Mode (Tab), with all points selected (which should be selected by default; A is the hotkey if they're not), move (G) along the z axis (Z) by 1 unit (-1).
   
5. Then, exit the Edit Mode (Tab) and scale (S) the cube. Note that the top remains on the XY plane. This is because object transformations are made in relation to the object's origin. When the points were all moved in the Edit Mode, the origin wasn't affected. So now, when scaling the top, because it's in line with the cube's origin, it remains on the same plane:
   
6. Exit the local view (NumPad /) and scale (S) the cube until it covers the bottom of the octopus body:
   

   In the Solid view (Z), it may look like the octopus has a flat bottom, but in the Wireframe View, (Z), it is clear that the cube is only hiding the bottom part. For the final model, the bottom needs to actually be flat:

   
7. Select the octopus body again, and in the Modifier tab, add a Boolean modifier:
   

   Note

   The Boolean modifier has the option of combining the selected object with another object in a number of ways; Intersect, Union, and Difference. Union joins the two object, so that they become one. Difference cuts the second object out of the first, and Intersect leaves only the part where the two objects overlapped. Boolean is a powerful tool and it's good to be aware of how to use it.

The following screenshot shows the three ways to combine selected objects with other objects:

1. Change the Operation value of the Boolean modifier to Difference. Click on the Object text field and choose Cube from the list that pops up:
   
2. Now the octopus body actually has a flat bottom that's suitable for 3D printing. This can be confirmed in the Wireframe view (Z):
   

The Boolean modifier is one of the most important tools in Blender that's used to create 3D printable objects. However, it can also be one of the most frustrating to use. The Boolean modifier demands that a mesh be clean and free from holes or problems. Even things that aren't obvious while looking at the mesh, such as duplicate points or flipped faces, can cause problems with the Boolean operation. If your Boolean operation isn't working, here are a few steps you can perform in order to try to fix it:

1. In the Edit Mode, select all the points (A), and in Tool Shelf (T), locate the Remove Doubles button in the Tools tab and click on it.
2. With all points still selected, switch to the Shading/UV tab, locate the Recalculate button, and click on it:
   
3. Now, exit the Edit Mode, and the Boolean modifier may work in many cases. If these fixes don't make the Boolean modifier work, then the best option may be to start over again, making sure that you perform all the steps correctly.