depends on what you're wanting to play really. For example if you want the dotted quarter note to stay the same throughout. In other words, a dotted quarter note from the 4/4 bar is going to be the exact same length in the 6/8 bar...time wise. This is the most common way of doing a 6/8 time change. If this is what you're wanting, practice the 4/4 bar with:
dotted quarter...dotted quarter....quarter note
This will equal four bars. Get the feel of that. Get used to the off beat rhythm. Now 6/8 is just going to be the dotted quarter notes....leave off the last quarter note. That's 6/8.
Practice going:
dotted quarter....dotted quarter...quarter (4/4)
dotted quarter....dotted quarter...quarter (4/4)
dotted quarter....dotted quarter (6/8)
dotted quarter....dotted quarter...quarter (4/4)
dotted quarter....dotted quarter...quarter (4/4)
dotted quarter....dotted quarter (6/8)
If you can get that, you'll realize that the eighth notes in the 4/4 bar are ALSO constant in the 6/8. So the dotted quarter note in the 4/4 is equal to 3 eighth notes in the 4/4 AND in the 6/8.
And 3 eighth notes equals HALF of the 6/8 bar. A full 6/8 bar is going to be 6 eighth notes (hence the name 6/8). Realizing this helps me feel the 6/8 easier. So I can go....
quarter.........quarter.....|...quarter.........quarter
eighth eighth eighth |eighth eighth eighth
quarter.........quarter.....|...quarter.........quarter
eighth eighth eighth |eighth eighth eighth
(the | denotes the middle of the measure)
hopefully all that lined up right on this post. See how in the 4/4 bar we have an extra quarter note? Essentially, since we are taking out a quarter note we have a 3/4 bar! However, the emphasis in a 6/8 is on each dotted quarter note (or eighth note if you want to feel it that way), while the 3/4's emphasis is on the quarter notes.
OR (maybe this will make it easier now that i just rambled on and on about all that) you can think of the 6/8 in two...which is just counting the dotted quarter notes.
eighth eighth eighth | eighth eighth eighth
...dotted quarter.....|.....dotted quarter.....
...........1...............|.............2...............
Okay, I hope this helped a bit
