Creating BMS for insta-travel:

Creating a BM seems straightforward enough that it shouldn't require any detailed explanations: draw a line from where you start (Origin) to where you want to end your warp (Destination), extend the line 15k beyond Destination. Use your scanner to get right there, bookmark your location and receive a seemingly perfect 15k insta-BM set to warp from Origin to Destination.
...or do you ?

Well, if you fly a Pod, a shuttle, or some other ship with near-zero inertia, you do.
Fly anything else and your mileage may vary greatly enough that your BM may prove unusable lest you warp at 20k instead of 15, and for this BM only
...headaches ahead.

The quick and easy way:

1 - Before warp, select (highlight by left-clicking) the object you come from (Origin) in your scanner list.
2 - Warp to 15k of Destination, and put the warp time to good use by setting your camera just on the 6 of your ship.
3 - Lock the destination object, and start approach.
4 - While approaching Destination, avoid bumping on the object by changing vector (preferably on the X or Y axis only) and pay attention to when and where on screen you clicked to change vector.
5 - Wait for the screen-wide cross-hair of Destination to disappear (it means you crossed its center) then click in roughly exact symmetry of your previous change of vector, to correct your course.
6 - Turn your camera angle 180°: you should see a small square box highlighting your origin (selected in scanner list). If not, make sure Origin is selected in your scanner display, then align your ship and Origin square box on camera.
7 - Open "People and Places", click on "add bookmark" and type in the name of the BM to set.
8 - Wait for Destination cross-hair box to roughly fall in line, which should conveniently happen around 10k, and click on OK in the modal dialog when at correct range (which varies, see theSize does matter section).

Bookmark done !

That's the gist of it, now there are a few things to keep in mind to avoid screwups.