I'll show you how to make a design cat tree.

For the price of a commercial and regular one, I could buy a sewing machine and a modest table saw, so I decided to design and build my own custom cat tree.
I'll also share with you the 3D model I made as my blueprint.

Tools:

Computer (for the design)
- Autodesk tinkercad or your favourite 3d free or paid software.
- I used Google Sketchup (free)
DrillElectric Jigsaw, a table saw could help you achieve better results, but is not necessary.
Sewing machine (optional but highly recommended)
Screwdriver (screwdriver bits for the drill HIGHLY recommended)
Measuring tape, Pencil, Ruler or something straight to draw lines.
Hardware stappler
Mandatory protection glasses and breathing mask. A couple of working gloves and ear protection will help you too

Materials or consumables:

Wood board (new or reclaimed)
2 Cardboard tubes discarded from fabric shops
Pallet wood or another wood wolution for attaching the cardboard tubes
Screws
Staples
Wood glue
around +200m of 3mm sisal rope
2 or 3m of fabric (depending on your design and fabric width)
A little cord for the ball toy
A plastic ball
Garment die of the desired color. (Optional. To die the rope)Thread

Step 1: The Design

Download and install Google Sketchup.
You can design your own or download this design I made. I called it GatiTower and I allow anybody to build it and modify it but forbid any commercial use of it or of it's modifications :)
Using google sketchup you can know the measures of every part.

It's measurements are based on my own living room furniture (that you can see modeled in 3d) using golden ratio wherever it was possible.

I warn you that the entrances ended being too small for my cats: I designed it when my first cat (european common) was under a year and not fully grown. Now I have 3 more, one is a maine coon and the other a persian (large cats), the holes are too small for they to enter comfortably (even if the maine coon still gets in and out because he likes to hide there)

gatitower Share.skp
Download View in 3D

Step 2: Gather Resources, Draw and Cut

The main materials for this project were reclaimed from trash as you can see. You can buy the wood board and replace the pallet with more wood, but I don't know how to find proper cardboard tubes away from a fabric shop. If you ask them they'll give it to you for free.

Step 3: Do They Fit?

Be sure to check if all fits where it should before cleaning all the mess from cutting wood. Due to the stange angles of this design, a lot of testing was done since I didn't have the proper tools to make perfectly angled cuttings.

The parts with a center hole can be made by drilling a wide hole and inserting the jigsaw blade from there as shown.

Step 4: Drill, Then Screw

Remember to drill a hole before inserting a screw so the wood don't break.

The cardboard tubes were attached by inserting pallet wood inside them and screwing it all together.

Have in mind that some of the pieces, specially the tubes must be screwed from INSIDE. That will decide your building order.

The triangular parts were glued. I also reclaimed the sawdust: With a little glue and sawdust you can make your own wood paste to fill any gap.

Step 5: The Side Pyramid

The side triangular form was made by gluing 3 layers of wood and cutting the excess to smooth it.

Then it was screwed to fix it in place. The cats use it as a step

Step 6: Dyeing and Cord Wraping the Tubes

Before of definitely installing the tubes, wrap them with the sisal rope.

If you don't want your rope to be it's natural color, you can tint it previously with some garment dye.

For this grey color I used black tint. It doesn't stain my cats or creates any paint dust when scratched by them, but have in mind that only light colors will be achievable (you'll end up with pinkish rope if you use red dye). Follow the instructions provided in the dye.
Don't unroll the rope before tincture as I did, it tangles a lot and is not worth it.

For the wrapping, staple the beginning of the rope in and end and start rolling.
I suggest you to screw to its half a long screw in the center of the pallet woord. Then bite it with your drill and use it as a spin motor. Someone can help you by activating the drill or guiding the rope, you'll have it all wrapped in no time.

Remember to add tension when wrapping, and to compress the rope downward so you can fit more of it. Your cats will compress it even more when naturally using it, creating not covered areas of cardboard tube. You can fix it adding more rope to the top section if this happens, so save the excess of dyed rope.

Step 7: Fabric Over the Wood

To give it a better look and feel we'll cover our cat tree with fabric. You can see the fabric that I selected, but remember that your cats love to climb, and they will hit their nails in the fabric while doing it, wearing it away over time. 8 months later of daily use there are parts with moderate wear with this fabric.

Notice that you'll have to apply fabric before screwing some pieces, like the posts: Staple the fabric in one end of the underside, screw the tubes, and then finish the stapling.

The complicated form of this design required some sewing. Good measuring and sewing skills will make for a better result (I just bought my sewing machine for this project, so I'm a newbie). Is like crafting a t-shirt: You'll fit the wooden part from the wider point, then staple that part to the underside or back where it won't be seen. Flat surfaces (simple boards) don't require sewing.

When fitted and stapled in the wood, cut an X in the openings and staple them inside to cover the hole wall.

Staples could scratch your wooden floor if you drag it around, plan your solution accordingly.

Step 8: The Final Touch

To make a toy ball I used a dollar store set of 3 balls (they'll play with the other 2 too), drilled a hole in in, passed a cord though it and knotted it to an eye screw hidden in the triangular stand.

The top part has a pallet wood insertion long and tight enough that doesn't need screwing it to the tube.