Tools you need:

welding equipment
angular grinder
dremel
tuner app
colors/paintings
old gas bottle

DISCLAIMER: Do the following on your own risk! I am not liable for any accidents happening.

First step (and MOST IMPORTANT!) is removing the valve of the bottle to fill the whole bottle with water!
Don't skip this, as it's really important to get _all_ of the explosive gas out of the bottle!

To remove the valve, we welded a long leverage so onw of us could remove the valve, whilst the other one kept holding the bottle.

Make sure to get the leverage as long as possible, as we need a lot of force ;)

And better do this whole water-thing outside to avoid scratches in yout bathroom and to keep the ugly smell from the inside of the bottle away from your flat!

Step 1: Time to Get Rid of the Parts We Don't Need

Now it's time to mark where to cut.

We want to have a clear line, so don't hesitate to use a ruler ;)
Then start with drilling a hole to insert your jigsaw. And always use enough cutting oil. As we're cutting steel, there will be a lot of heat at the tip of our drill or the saw blade. If you're not going to use enough oil, your tools will be dull very soon.

The outer rim you can remove using an angular grinder.
Always make sure to wear eye- and ear-protection!

Step 2: Remove the Valve's Socket

Easiest way to remove the valve's socket is using the angular grinder, again, cut it off and make a clear circle with the jigsaw, later.

As you can see on the second picture, we used some screws in the wooden board to fix the gas bottle while working on it ;)

Step 3: Remove the Smelling Coating and Painting

On the inside of the propane bottle if an ugly smelling coating (which might be a sideproduct of the smelling additive every butane gas container has to make sure everybody can smell if there's a leakage).
Use the angular grinder, again, to remove the smelling stuff and to get a blank surface for the primer later.

Step 4: Draw Tongues

Now it's time to draw the tongues on your drum.
We printed out a pattern we found online (can't remember the source...but you'll find it easily using you favorite search engine - or have a closer look on our copy, at the bottom is the author's URL).
Then we used a cutter to copy the tongues to the surface of our drums (we created little scratches and after that we re-drawed them using a fine permanent marker).

Then we used some cheap dremel-like tool to grind a first slot in every tounge.
But make sure not to cut at the end of the lines, as we don't know how long the tongues will be later.

Step 5: Time to Grab Your Jigsaw

Using the jigsaw, we cut the whole tongues.
But beware: Once the tongue is too big, it's a lot of (soldering & grinding) work to undo it.

Cut both sides of the tongues alternating and as you get closer to your tone (smaller slots = higher tones, bigger slots = lower tones) start being more and more careful!

We cut half a millimeter too far, so we had to drop down the whole tuning of our drum.
That's a lot of work....and every millimeter might involve the risk you cannot tune the drum anymore in the way you want.
If the tongues are too small or too big, they'll not sound pretty nice anymore!

Use some rubber coated handlebar as mallet. This way you'll avoid hearing only high tones when you beat the tongue.

We did tune the drum higher than we need, as we wanted to weld the bottom to the top of our drum first and then fine-tune the whole thing ;)

Step 6: Weld Both Hemispheres Together

Now it's time to weld both hemispheres together.

It's easier if you cut an oval into an old board which fits the outer contour of your tankdrum, rotated by 90 degrees.
So you can weld, remove the bed ash with a hammer and grind the welding spots more easy.
Without this board it would have been a mess!

The better you prepare the weldingspots now, the better the surface will be later.

Btw: As we did this with a looooong break (some month...we forgot to go on :D) we painted the whole drums with primer.This way we avoid any strain and rust.

Step 7: Fine-tuning

When the welding is finished, wait till everything is cooled down.

Then start the tuner app you like on your phone (or maybe grab a hardwaretuner or some tuning forks...what ever you have and like) to start the finetuning.
We started to write down each note on the tongue, to get a better overview.

On some tongues you might see crossed out notes on the pictures....that's where we failed and had to go on with a lower note (or start welding and sawing, again). In this case we wanted the d-minor pentatonic for the first drum. Because of a little bit of material taken off too much we decided to go for the c-minor pentatonic. The relations are the same between the notes, but everything is lower 2 semitones compared to the previously thought d-minor-pentatonic. So no problem, everything always works out at the end ;)

I recommend to go for a pentatonic set of notes (major or minor), in order to get an easy to play instrument. But you can choose whatever you like. Why not go for dorian, or a gypsy scale?

Hint: Sometimes it's really helpfull if you put some tape or a hand on the tongues you're actually not tuning.
If all tongues oscillate at the same time while tuning, it's really hard to hear what's really going on.

Step 8: Painting

Take some color you want and start painting ;)

Step 9: Add Details to Your Painting

It's a good idea to add some details to your design before using clear coating.

Good luck we have a cutting plotter here, so it was easy to create stencils ;)

Step 10: Finish the Painting...

...with some clear coating ;)

To avoid scratching our finish too fast, we added some clear coating on top of our painting.
Make sure your drum isn't in touch with anything whilst the coating is wet - else it will stick on it and you can start from the beginning :D

If you like, you can add some rubber foot. There would be a perfect number, as it won't wobble on any ground then.

Step 11: Build Some Cheap Mallets

Sometimes I like to play the tankdrum with my fingers, sometimes I like soft mallets.
As you can see it's easy creating some basic mallets - just use some tape, a tissue and chop sticks ;)