To do this, go to File > Export > Texture Editing > 3D Model with UV Info.

First you will want to delete all the objects in Blender's default scene. To do this, press A until everything is selected, then press X to delete them.

To import the OBJ file, go to File > Import > Wavefront (.obj). Select the file you exported in step 1.

Select the group by right clicking on it.

Move it to the center by dragging the arrows on each axis.

You can also move things by pressing G.

To make things easier, you may want to reset the object's origins. To do this, go to Set Origin > Origin to Geometry.

At this point you will probably also want to save the model with CTRL-S.

In Pepakura Designer, go to 2D Menu > Change Scale > Set Scale.

Copy the scale value.

In Blender, right click on your model to select it and then press N to bring up the Object Transformation panel.

Change the scale of all three axes to what you copied from Pepakura.

Select the object and then switch to Edit Mode.

Press A until everything is deselected.

Go to Grease Pencil > Ruler/Protractor

Hold CTRL and drag the mouse from one point to another.

Press ESC to clear the measurements.

Right click anywhere in the 2D view and select Measure Distance between Two Points.

Now you can click on any two points in the 3D view to get a measurement.

Select the group and switch to Edit Mode, if you are not there already.

Press A until nothing is selected, then right click on one piece to select one of its vertexes.

Press CTRL-L to select the entire piece (all linked vertices).

Press P then choose Selection to separate that piece from the group.

Select a piece and go into Edit Mode.

Switch to Face Select mode.

You can select faces of the support structures by right clicking on them individually, but I find it easier to use Circle Select.

Press C then click to select faces touching the circle.

You can use the scroll wheel to increase/decrease the size of the circle.

When you are done selecting, press ESC.

To delete the faces, press X and then select Faces.

Tip: You can hide the pieces you aren't working on right now by clicking the eyeball icon next to them in the Outliner.

Also remember to save your progress as you go (CTRL-S).

The easiest way to do this is with the Solidify modifier.

Select your object, then go to the Modifiers tab of the Properties editor.

Click Add Modifier and then choose Solidify from the Generate section.

Increase the thickness to whatever you think is best. I think 3.0 is a good starting point.

If you want to make the changes permanent, you can hit Apply. This is not really necessary, though. You can leave it as is, which will allow you to edit the modifier later.

Switch to edge select mode and select one of the outer edges.

Go to Select > Edge Loops to select the rest of the ring.

Press ALT+F to fill in the back with polygons.

Go into edit mode and press A to select everything.

Press E to extrude. This will make a copy of the mesh that is linked to the original around the edges. You can move this copy around with the mouse, but in this case we want to keep it in the same place so just press ESC.

Now press S to scale down the copy. We are going to turn it into the inside surface.

We want the inside of this tube to be just as tall as the outside, so press SHIFT+Z to tell blender not to scale the Z axis. Conversely, you could press Z if you wanted to scale only the Z axis but not any others. The same goes for the X and Y axes as well.

Move the mouse until the tube is the right thickness, then press ESC when you are done.

You can use ALT+B to slice open the mesh and see inside of it.

Here you can see a part of the inside that's sticking through the outside. I'll need to fix this by deleting faces and moving things around.

This part is also very thin and might not print well.

You can also switch the view to wireframe mode, to make everything transparent.

We need to select the line where the cut will be made. Select one edge and then keep selecting more using SHIFT or CTRL.

CTRL makes it easy by selecting the shortest path between the first line and the one you click on.

Press V and ESC to rip the mesh along the line you selected.

Press CTRL+L to select the entire linked section, then move it away from the rest.

Press P to ungroup it from the original.

Select one of the edges of the open area, then go to ``Select > Edge Loops".

Press ALT+F to fill in the area.

Select 3 points, then press F to make a face from them.

Select all then go to Mesh > Normals > Recalculate Outside. If you see any faces change color slightly, it means they were reversed before.

Go to Select > Select All by Trait > Non Manifold.

Any parts of the mesh that are now selected have issues. You will need to manually fix these. The best way is to probably delete those faces and reconstruct that area.

One thing that often leads to non manifold geometry is vertices which are very close to each other but not actually linked. To fix these, select all and then go to Mesh > Vertices > Remove Doubles

To smooth things out we are going to use Blender's Subdivision Surface Modifier. Open to the Modifiers tab and then go to Add Modifier > Subdivision Surface.

The setting you want to play with now is the number of subdivisions (View, not Render). I generally like to do 3 or 4. The more subdivisions, the smoother the surface. However it will also make more polygons which means larger file sizes and slower performance.

Lets start off by temporarily disabling the Subdivision Surface modifier. You can do this by clicking the eyeball icon.

Now lets select the edges we want to crease. An easy way to do this is by going to Select > Sharp Edges. This will select all edges with a sharp angle.

Play with the sharpness threshold until it looks like it has most of the important edges. It does not have to be perfect. We can add or remove more later. This is just to give you a good starting point.

Crease the edges by pressing SHIFT+E then type 1.0 and press ENTER.

To uncrease an edge, you would type -1.0 instead

You can now unhide the modifier to see how it looks, then go back and crease/uncrease more edges as needed.

Thats it! You now have a model thats ready for printing. Go to File > Export > Stl (.stl) to export the currently selected object.

Now bring it into your favorite slicer and start printing.