fab;
Here is a snippet I posted some years ago with the basics:
An Easy "shunt" is a 1 foot long piece of #10 solid copper wire will show 1 millivolt [.001 volt] drop across it for every amp thru it. Not the most precise method but perfectly acceptable for a good idea of the current. I use several wire shunts in my setup. I calibrate mine against a factory shunt / meter combination but you don't need that.
Anyway, if you like to tinker this will work.
What ever happened to "use what you got"?
Anyone can buy a meter / shunt combo but where is the fun in that?
If it was me and it isn't
I would start with a hunk of copper wire a couple feet long and drive a known current through it with a battery charger with an ammeter, whatever attach the wire in the battery + lead. Then attach one lead of your meter to the end nearest the charger ( + in this case). Then test the - lead at different points along the copper wire. With the right length wire you can set the meter to read 1/10 of the current. If the meter reads backwards, reverse the leads.
If you like to tinker anyway.
Don't let the "buy one" crowd make you think you need factory shunts. They are easy and do work but there is more knowledge to be gained in doing it your own way.
Just my opinion.
I am using factory shunts now but only because they showed up in my junk pile.
Tom