It took me a week or so, though I still get the occasional fur shot if I am in a rush. Here's the technique I use: I usually target Ninja's sides or upper back -- wherever I can get a grip on the most skin. I pull up A LOT of fur and skin - using the thumb, index finger, middle finger and ring finger of my LEFT hand - and I pull it up about an inch high, into a big 3-D tent -- then shoot into the lower third of the tent that I have created. If you shoot too high you will get a fur shot, as the needle will come out the back side. And if I try to create the tent with just two fingers, it is often too shallow (so the needle comes out the back).
I also now hesitate before pushing in the plunger (for just a second) which gives me time to assess whether I placed the needle low enough in the tent or not, or if my tent is too shallow. You will get a feel for whether you are in the right place -- and if not, you can withdraw the needle and try lower before you push the plunger in. Ninja doesn't seem to mind.
If the fur does not feel moist near the injection site and your cat's fur does not smell like "old bandaids" (LOL to Rhiannon!) afterward, then you probably got the shot in ok.
I do still have the occasional fur shot, but mostly it is when I am in a hurry, or do not have adequate lighting for the task. Adequate lighting is really important since you have a black cat like mine -- and yours has longer hair, which makes it even harder for you.