You gotta couple different options.
Through the dip stick tube, basically, you want to find a tube long enough that will slip down into the tube, then siphon away...
Take off oil filter, take sample from oil filter....I know Blackstone makes a note, if you DO go this route, that you TELL them you took the sample from the oil filter....I don't see why...all oil is filtered through there anyways, I guess there's a possibility you could take some of the "filtered" contaminants (and thus, not a "real" danger to your engine...) in with your sample.
But you'll want to re-fill that oil filter, so you don't get the "dry start" affect....due to no oil in the filter when it cranks....
If you go this route, at the END of your OCI, I would consider taking the sample in the same way, so you have an "apples to apples" comparison, mid-OCI and post-OCI with the sample from the oil filter.
This is probably really the easiest way to get a sample mid-OCI....and really, can't see any harm in it, since you will be "topping off" the filter capacity with "fresh" oil, it'll slightly skew the TBN if you desire to get the TBN tested (which I'd assume, since you are looking at extending the OCI, and the TBN is basically how much "life" is left of the oil...).
And last (in order of preference) would be through the drain plug
But that'll take some "skill" and quick reflexes methinks, to pop a quick sample, and then quickly re-tightening the drain plug without stripping threads, etc
I've heard about the easiest is with the siphon method, and many of the labs sell their own "pump" to collect the sample from the dipstick tube; but I just can't quite grasp it, perhaps they got some high-temp grade plastic or smthng, but I don't think I'd try that route with the $3 "bulb siphon" from Harbor Freight/Walmart
I'd hate for that plastic tube to "melt" inside the dipstick tube as I am grabbing the sample from a warmed up engine.....since you really want to get the sample after the car has had time to warm up, and coolant/moisture/gas has been hopefully dissipated - unless you got serious leakage somewhere