In another post I wrote about orienting yourself to the Arduino pins using tactile landmarks such as the gaps in the left and right header blocks. Unfortunately, the header blocks on some Arduino boards are manufactured without gaps between sections. Instead, both right and left headers are continuous blocks that include both upper and lower sections. Where the gap would be on most boards is simply a double-wide space on the header with no hole at all.
You can orient yourself the hard way using a probe such as a jumper pin, wire, or stylus to feel your way along the top of the header to find the spot where there is a wider gap between holes. This is the break between the upper and lower sections. While this is doable, it’s definitely a pain to do every time you need to find a particular pin.
To make navigation easier on these notchless boards, use a serrated dinner knife or a metal nail file to cut a notch between upper and lower sections that is easily detectable by touch. Guide the knife or file by placing male jumper cables on either side of the gap, that is, in the bottom hole of the top section and the top hole of the bottom section. Placing the knife or file between the two jumpers, gently saw back and forth to cut a small notch in the plastic header. Be sure to hold the Arduino by the header, not the board, to avoid straining the joint between the header and the board. The notch doesn’t need to be deep in order to be easily felt. Five to ten strokes with the knife or file should be enough. Check to be sure you can feel it and continue if necessary.
If you don’t want to permanently mark your header you can use a probe to find the gap and place a temporary landmark such as a flexible plastic toothpick or male-female jumper cable in one of the adjacent holes. The main consideration here is not to use anything which might accidentally break off or create a short.