Depending on the needle valve you have, there are several options available.
For example, in my particular setup, I have a 1/8" male fitting on my metering valve. Thi screws in perfectly to my 1/8" female fitting on my JBJ bubble counter. On the other end of my bubble counter, I have a compression fitting that fits into the tubing as normal.
Some people like to skip the bubble counter, and attach either a compression fitting or a hose barb fitting directly to the needle valve. This method also works.