Alright, so I just got a PSP Slim and its version is already 3.80. Anybody got any ideas on downgrading then custom firmware'ing it up?

You should be able to downgrade it with a Pandora's Battery.

Can you enlighten me on what a Pandora is and how to get it?

I copy/pasted this from another post I made a while ago:

Pandora's Battery is the name given to a PSP battery which essentially has been converted (through changing the serial number stored in its EEPROM) to boot the PSP into service mode. In this mode it can launch boot code stored on the memory stick, allowing the PSP to be unbricked, downgraded or (as you require) updated to custom firmware.

The main ways of doing this are:
- someone with a PSP already running custom firmware using homebrew to convert a battery for you
- hard-modding the battery by cutting a specific pin inside it
- buying a pre-converted Pandora's Battery, or a Datel TOOL battery

Then you need to set up your memory stick with the boot code, of which there are two main ways:
- someone with a homebre-enabled PSP setting up your memory stick for you
- running an application on your PC to do it

There's more info on the method of doing it without having a homebrew enabled PSP here:
http://forums.afterdawn.com/thread_view.cfm/591203

And the initial release notes of Pandora's Battery might give some background information:
http://www.noobz.eu/joomla/news/pandoras-battery.html

Wait, I'm confused. For the pandora's battery, I need CFW or 1.50 FW. And to use the time machine from Dark Alex to turn mine into 1.50, I need a Pandora's battery. Solution please?

You could buy one that's already made. They sell them on sites like eBay if i'm not mistaken.

Is it possible to make my own? I'm low on money just from the PSP and memory card.

Yes, it's possible. You need a PSP with CFW on it though so if you don't know anyone who does you may have to wait until you save up and can buy one.

This is not strictly true; you can make one with just your PC as long as you are willing to hard-mod your battery:
http://forums.afterdawn.com/thread_view.cfm/591203

If you choose to do it this way then I'd recommend you install UPMS so you can also use the battery as normal, but that's after you hard-mod your battery and install custom firmware.

I get an error whenever I use that. =/

EDIT : I got v3.71 M33, but it won't turn on whenever I put in my Pandorized battery.