You need some reference note to get you started. Middle "C" on a piano will give you the third (C) string note.
Once you have C tuned, fret the 3rd string at the 4th fret and you have a reference note to tune your 2nd (E) string. Once you have E, fret that string on the 5th fret to get your "A" note.
Now, turn to the 4th (G) string and tune it so the 2nd fret 4th string matches the open A string you already tuned.
That's it! There's more room for errors as the relative notes depend on the accuracy of the other ones you tuned rather than a solid reference note, but people have tuned that way for years before electronic tuners were invented.