Welcome to the community!
Good advice has already been posted about the tool you need. (I already had an angle grinder, and I Amazon’ed my diamond blades for under $10.) The other piece of your question may be the relative complexity of fitting firebrick “parts” into the larger herringbone pattern.
Unlike with the standard bricks around your frame, and with the firebrick that will form the arch of your oven, the firebrick hearth is fit as tightly together as you can manage. They will be held in by the frame, and you want them to have little or no joint space between them.
When you lay out the herringbone (which by the way is completely FUN!), pay attention to the angle and level each brick on the sand underneath it. A mason’s line across the top, set at the 45° angle, would be helpful to get you started.
You need to get a uniform surface that is flush to your brick frame. Lay as many whole firebricks in the herringbone pattern as you can, then ensure that the entire pattern is flush and angled correctly. You’re not mortaring them in place, so you are free to redo it if you aren’t happy with it the first time.
When you have all the whole firebrick in place, you can start nipping pieces of firebrick into the voids that are left. Start by taking a firebrick and laying it where it would fall in the pattern, using the brick frame to support it. Use a Sharpie or crayon to mark the spots where it touches the frame, and a combination square to extend that line to the top of the firebrick.
Once you’ve marked the two sides, use the combination square again to draw a line on the top between the two side marks. That’s your cut line, and you’ll be able to follow it all the way down because of the side lines.
Don’t worry about the lines themselves—you’ll be cutting them away with the grinder, and any remnant will end up under the walls of the oven.
DO worry about making sure each cut piece is level with the rest of the hearth and the brick frame. Again, that’s what the base sand is for.
And finally, when you see a reference to Step 48 in the instructions, ignore it. There is no Step 48, and no need for a small bag of flour. The idea is to wedge your hearth brick in tightly, and any remaining space between the firebrick will be filled in with ash from the delicious pizza you’ll be making.
Hope this helps!