Hi Neilda, I found the greasing issue a bit of an issue when I got my car about a year ago. I realigned the nipples, using fibre washers as spacers, so that I could attach the grease gun without removing the wheels.
I bought a flexible tube to attach to the grease gun easier to put on take off.
And I use Nitrile gloves to keep my hand clean. This prevents me getting grease on the car/brakes etc.
I grease the bottom first, the clean off the excess and then I put grease into the top nipple and then clean off the excess.
What you need is a friendly owner to show you how to do it and then you will be fine.
