If there is an unused port into which you can thread a hose barb fitting that would be best. Otherwise you install a "T" fitting in the tall tale line. Some engines that works while others it doesn't. A new impeller should be installed when you install the guage. You then note idle, mid range and wide open throttle water pressure readings. If pressure begins to drop over time you know the impeller is in need of replacement. If you don't do that you really don't know what "normal" is for that engine.
Here is a tell tale installation on a 75 HP Merc.