While difficult to tell for sure, that modchip appears to be similar to the Stealth Elite modchip modchip.ca (modchip.us) used to sell before they shutdown. The product from modchip.ca was always reliable and worked well, although all of the image links on that site appear to be broken. A quick search leads me to believe the Stealth Elite modchip was actually the more common MM3. The first site you mentioned has the MM3 hex files and install diagrams. While they appear to still sell pre-programmed chips, I’ve heard that shipping is from China and slow. Just remember to make sure whatever option you choose is compatible with your PSX region/model and that you use the install diagrams from the site where you buy the chip or download files. There are a number of different PSX modchip offerings, and each used its own wiring scheme.
I took a look at the install diagram that you mentioned and it appears to be at least as good a quality as you can hope to expect. If you are having difficulty interpreting it, try enlarging the image slightly. The #2 is pointing to the “top left” pin of a chip, #3 is pointing to the “left” side of either a resister or capacitor, and #6 is pointing to the “top” side of either a resistor or capacitor.
When installing the modchip, just go slow, make sure you are making solid connections with no shorts, you don’t damage anything, and you don’t leave wire clippings or small pieces of solder loose in the system. Additionally, the first thing you should do is plan on where you are going to mount the chip and how you are going to route the wires. Try to not cross any wires or have them cross over an existing component. The PSX is a relatively easy console to chip, so you should be fine, just go slow and be careful.
