How do I choose a tonearm cable?

A  adapter / connector the same or similar to the one in the Link will enable an increased amount of Cable options to be considered.

These adapters can be found with more expensive design which are using Pure Copper for the Signal Path.

I have also seen a DIY design built using Pure Copper connectors and connecting wire, built into a small enclosure for approx' £50. These designs are available built, but will be found close to £200 if from a Brand.       

https://www.hificonnexion.com/din-to-rca-connectors

I really like the Cardas tonearm cable.  I've moved away from their interconnect and speaker cable, but all my tables still use Cardas DIN to RCA tonearm cable.

Just a personal preference, but I haven't found anything I like better.  VERY well built and sound great.

For my 3rd arm, front left corner mount,

I needed an 8’ length, and the din connector in the arm tube was recessed, so my din connector had to be a ’long neck’ without a shoulder/wider tube. I could not find anything ready made long enough.

I found a long neck shoulder connector, gave the connector info to Pine Tree Audio,

https://sites.google.com/view/pine-tree-audio/custom-shop-work?authuser=0

and they made me a custom cable for not too much money.

meanwhile, while waiting

I bought a cheap $10. Phono cable from Amazon to use while waiting for an expensive Ortofon Phono Cable that my Russian tonearm maker insisted I get to use with it.

https://ortofon.com/products/6nx-tsw-1010

This one is 10’ long, $15.

https://www.amazon.com/Ground-Turntable-Interconnect-UIInosoo-Amplifier/dp/B0978Y1KY9/ref=sr_1_4?sr=8-4

You can use the din/rca adapter shown above, at least temporarily. Then compare any ’better’, custom cable to it, at least you will know if it makes a difference you can hear.

My 3 arm’s cables go to my SUT with 3 selectable inputs, and it’s output cable goes to my preamp.

https://www.audiogon.com/systems/11318

I couldn’t hear any difference. I’ve skipped the SUT, used the various phono cables directly to my preamp’s MM input, switched them, ... I do not hear any difference from my variety of phono cables!

I can have 3 arms/cartridges playing simultaneously, preamp has phono1 and phono 2, identical MM phono inputs, I can compare instantly

I hear differences of cartridges, I hear impedance load changes, and I hear tube change differences, and friend’s various equipment differences, but not cheap/good/best phono cables.

btw, you can always make a separate ground wire, standard green coated wire, nothing special needed, if you want to use any interconnect cable without a ground

Here’s how I approach it. First and foremost, set your budget. Sky is the limit so it’s better to have a guideline to narrow things down. 

Next is to set a strategy: 

1. Best you can find from a reputable company on the used market. 
2. Something recommended from a friend that you’ve heard used in a system you like. 
3. If your TT manufacturer uses a specific cable, consider going with that brand for continuity. Ex: Mofi uses Cardas. VPI often uses Nordost. 
4. Stick with the cables in your system if you already like them. 
 

Once you have a budget set and pick one of those strategies it narrows the field pretty significantly and should give you confidence moving forward. 

If 12 or 10 gauge wire is good for speaker wire, it’s good for any connection. Of course, no matter the wire, there’s no objective difference in the sound.