When I first got my pink Platinum Preppy Fountain Pen I was very nervous about putting the cartridge in. I didn't want to push it so hard that it ruined the pen, but I also wanted to make sure it was attached securely so the ink wouldn't leak and ruin my purse. After getting some feedback I realized that I wasn't alone in my fears, so I decided to share what I've learned about fountain pen cartridges to help others calm their fears about putting their pens together.
First of all, the tension that you feel when pushing the cartridge into the pen comes from the stopping device at the top of the cartridge, meant to keep ink in prior to assembly. In the Platinum Preppy Fountain Pen Cartridges this is a small metal ball, in standard cartridges (like J. Herbin Universal) it is a tiny glass ball, and in LAMY cartridges it's just a thin layer of plastic. When you push the cartridge in you are not only attaching it to the pen, you are pushing the stopper device in and starting the ink flow.
The way to put cartridges in is fairly simple: slowly and gently while ensuring a firm attachment. Put it in straight on and start pushing gently. When you feel the tension from the release of the stopping device it should be just about in, and a tiny extra push to secure it won't hurt.
Once the cartridge is attached to the pen it takes a little while for the ink to flow through the feed to the nib. Allow your pen to sit for awhile before you try to write with it. If the ink doesn't seem to be getting to the nib, let the pen rest with the cap on and the nib pointing downwards. If you're still having trouble getting it to write you can run cool water over the nib or pull the cartridge out and put a couple of drops of ink on the nib to encourage the ink to flow.