You can make the tags the same way as in the Mad Hatter's Hat Tutorial - roll the paste out very thin so that you can see through it; place a printed out template underneath; paint the message on. For details go to the Mad Hatter's Hat Tutorial
If you prefer to use a marker, you can cut the tags out, leave them to dry out completely and then write the message on. I like to use the fine tip marker.
I first make the letters very thin. If I'm not happy with the positioning or the spacing of the letters I adjust them by widening them (f.ex. if the letter seemed to be too far to the left I widen it slightly to the right).
You can also find a font with letters which are not all in one line - then if the letters written by you are not even it won't be that visible. I recommend Google fonts - you can browse through hundreds of fonts and check how the message would look in a certain font.
I suggest making 2-3 tags just in case they break ;)