i use my ears and an L2 plug

I am not an accomplished mastering engineer, but I like to use an RMS meter (and my ears of course).

Do you really want all the tracks at the same volume? Do they sound best that way? If played live, would the band play them that way?

Ears.

Use your ears and judge for the best apparent volume. Like David says, an RMS meter will be more appropriate for this purpose but the ears has the final word.

Cheers!

wait til you have a reason to drive for a hour or so. burn a disc, take it with you in the car. If you can listen to the disc at a reasonably low level with engine/traffic noise distracting you without having to adjust the volume after every track, then you're in the ballpark.

if your music isnt solid blast metal or similar, you might compare parts to parts. say one is solid distortion through the whole song. the other has soft parts and loud parts. the other is pure soft. match the loud parts in the middle song with the loud one, then the soft one with the soft parts in the middle song. using RMS and your ears of course. then play through each 3 to see if they match up, then play the hard against the soft. this is assuming the middle song is mixed properly and there isnt a major dip in volume on the soft part.

if you match RMS of a loud song vs a soft one you are probably gonna end up raising your soft song far beyond what it should be. which is why you cant rely solely on RMS.

the more diverse your songs are composition wise, the harder itll be. matching a techno vs a metal vs an acoustic pop song is al ot harder than 3 metal songs in a row.

id suggest your loudesst first, else if you do your softest first, by the time you get to your loudest you might find you are oversquishing it and have to go back through all of them again.

Use an RMS VU meter + a peak meter, a compressor, an EQ and a brickwall limiter. AND use your ears.