Yeah, one of those A23 12V batteries should work okay for testing. And those aren't capable of producing anywhere near the amount of current as a car battery so you'd probably be okay to skip the fuse.
To make the test connections, clip one of the test leads to the +12v terminal on the back of the gauge (I usually use a red lead for this). Clip another of the test leads to the sender terminal on the back of the gauge (I usually use a green lead for this). And clip a couple of test leads to a convenient spot on the metal cluster housing (I use black leads for this). Now clip the other end of the green (sender) lead to one side of a resistor, and one of the black (ground) leads to the other side of the resistor. Finally, connect the other black (ground) lead to the negative battery terminal and the red (+12v) lead to the positive battery terminal.
Like this:
[+12v terminal on gauge]-----(red test lead)-----[positive battery terminal]
[sender terminal on gauge]------(green test lead)-----<15 ohm resistor>-----(black test lead)------[metal cluster case]
[metal cluster case]-----(another black test lead)------[negative battery terminal]
To put two resistors in series you can just twist their leads together and treat it like a single resistor. Just be sure nothing else makes contact with the intermediate connection between the two. Or as an alternative, you can use an additional test lead to join the two together.
Like this:
[sender terminal on gauge]-----(green test lead)-----<15 ohm resistor>----(another green test lead)----<another 15 ohm resistor>-----(black test lead)-----[metal cluster case]