Hi, MakesMeSmile. There are a couple ways of doing this.
Some go by syllables...but that's more of a poetic way of doing it...more of a poetry writer to lyricist conversion. I remember your lyric "Empty Eyes" and enjoyed it...so it does work.
For me, I usually have a certain set rhythm in my head...I count out stresses...sometimes every line will have 4 stresses, or 3 stresses, or 2...depending on the lyric. The chorus and bridge obviously can be different. Sometimes I play around a bit and mix things around to try something new. Rules were always made to be broken, but within reason. Anyone just beginning should be comfortable with the rules before expanding. I remember with "Empty Eyes" you didn't have a problem with meter and I felt a rhythm to your lyric, so you might want to have fun with it and try it both ways :-)
Keep in mind, others critiquing your lyric may have a different rhythm in their head than you do. For the title Someday for instance...one word...some might sing it a SOME-day, some-DAY or SO-ome-DAY or SO-ome-DA-AY, kinda being extreme here, but to show you how one two-syllable word actually can be four syllables with different people having different stresses when they read a lyric.
Hope this helps.
Jena