Couldn't check specs for the one you are ordering but make sure it does BOTH AC and DC amps...many of the really inexpensive ones don't...even thought they measure voltage both ways. The cheapest AC/DC amp clamp meters I've found are in the 50-60 buck range on Amazon.
As to where to use it...just clamp it over your red main battery wire to measure DC amps parasitic load...or the load of any particular piece of equipment when you switch it on.
When clamping...just make sure to clamp a SINGLE wire...not both the +/- . This is sometimes a problem with dual conductor AC wire...and this little device is what you can plug into...which effectively separates the wires to enable you to clamp without cutting!
Amazon.com: Auto ranging AC/DC Digital Clamp Meter: Home Improvement
The above link is for the meter I use now in case the one you're looking at is not ac/dc for amps.