Relax, it's easy. Just read through my post, and follow the steps:

To set it up:
1. You need to have a Skype account (skype.com).
1.a. You need to have an active internet connection on your N900.
2. Fire up your N900, open the menu, go to "Settings".
3. In the "Connectivity" section, select "VoIP and IM accounts".
4. Press "New", select "Skype".
5. Enter your Skype Name and password, and press "Sign in".

You're good to go. If everything was OK, it will briefly log in to Skype and copy all your Skype contacts into your contact list. You can see this in the Contacts application.
If, by any chance, there are some contacts which you have in Skype and also your "regular" address book, you can merge them together. (Open a contact, press the title bar to open options and click "Merge". )

Then, follow these steps for your Skype usage:
1. Fire up your N900, make an internet connection.
2. In the status bar, there will be a button "Availability".
3. Press that, and then select "Online" and press "Save".
4. You will see a little flashing green circle at the status bar. If it remains constantly green, you're logged in.
5. Open Contacts
6. Note that those who are logged in to Skype have the same green circle in the contact list.
7. Select a contact who has Skype (and is online), and press "Skype call" to call him/her, or "Skype IM" to chat with him/her.
8. Nice chatting!

Try it out and I hope everything will be fine. It works very well for me.
Note that you'll need to hold the device to your ears when you make a Skype call (just like a phone call).

Step 42: Press the "Thanks!" button below this post to say thank you to good old Venemo.