You hover over the 'My Gaia' tab that's right next to the 'GCash' tab. To the very right in the tab should be 'Mail', click on that. After clicking on that you'll notice to the very top left under that little bar that has more tab is a 'New' icon. Hover over that and you'll get the choice between Private Message and Text Message. Logically you should click on Private Message. After that you should be taken to a page that has the layout of how it would be to send an email.
Here's practically how you should do it.
1. Enter the name of the person/persons you wanna PM in the 'Username' box.If you were to put more than one person you would have to separate the names with commas. For example " ShitL0rd, W4ffl3snP4nc4k3s, Rekkit ".
2. (Optional) You can enter in a subject. For example if you were inquiring a seller about an item on the MP, put the name of the item.
3. The most important part or maybe not the most important part; The message. It's pretty simple just type the message you want, put emojis, change the size, color, etc of the text.
Extras: after the message you're given the option you add your signature. For me I only do that if it's someone I'm negotiating with but, that's me. Do whatever floats ye boats M8ies.
After all that is done you can either click 'Cancel' which will delete the message, 'Preview' to see what your message will look like after it is sent. Lastly 'Send' Which will forward your message to person/persons of interest.
After clicking send you should receive a message stating "Information, Your Private Message has been sent to: Shitl0rd (example ppl), Click here to return to your Inbox" If you wanna make another PM just click the blue 'here' text, if not then just do whatever things you were doing before.
but yeah, you have now learned the ways of the Private Message young grasshopper.