The slim psp cannot be downgraded to firmware 1.50. The only way to get custom firmware from version 3.95 is to use the pandora battery and magic memory stick.
Requirements
a) PSP Slim (PSP 2000) or PSP Fat (PSP 1000) and PSP USB cable.
[Note: A card reader can also be used in place of USB Cable]
b) Pandora's Battery.
c) Memory Stick Pro Duo [64mb - 4gb] I have tested this and can confirm that it works with fake sticks.
d) PSP Grader Lite v003
e) Official FW Update 3.90
How To
Step 1) Install PSP Grader to your PC in the default location [C:\ ]
Step 2) Run PSP Grader, you will see a boxe at the top, add the location to your Official FW Update 3.90 into the corresponding box.
http://v2.x-projects.org/tp-images/Image/1212784043Graderv003.png
Step 3) Plug your PSP into the PC via the USB cable ensuring that the Memory Stick Pro Duo is inserted into the PSP. Also be aware that this is going to format your memory stick so make sure to back up any important files first. The PSP Grader program will find your PSP and ask if it is the drive you wish to format, select YES. Now in the bottom right corner of the PSP grader program select start.
Step 4) Your Memory stick is now ready, unplug the PSP from the USB cable, remove the battery and insert the Pandora's Battery whilst holding the L should button on the PSP. Once the IPL has loaded on the PSP simply press the X button. Wait. After around 1 - 3 minutes the Flash shall be done, press X to shutdown the psp, it is now safe to remove the pandora battery. [Note] Its is very important to just wait and not switch off the PSP or remove the Memory Stick or Pandora's Battery, it could result in a brick if you do not wait.
Step 5) Now you have custom FW 3.90-M33-3 on your PSP. You may now format your memory stick, you're done.