This is a mneumonic device to memorize the Aufbau principle. It is based on memorizing a simple number sequence and visually associating letters to the numbers.


Start by visually remembering “the initial pattern” – 6 numbers as written below:

3
4
45
56

Associate the letters D and FD.
A single number gets the 1 letter (D), a double number gets the 2 letters(FD):

3 D
4 D
45 FD
56 FD

Surround each number with a “+1 number” on both sides.
Use numbers 4 to 7:

434 D
545 D
6456 FD
7567 FD

Visually recognize where “the initial pattern” fits in the following:
1
22
33
434       <——3
545       <——4
6456     <——45
7567     <——56

Associate the letters:
1
22
33
434       <——D
545       <——D
6456     <——FD
7567     <——FD

For a visual of:
1
22
33
43D4
54D5
64F5D6
75F6D7

The final rule is:
Every line starts with S and ends with P.

1s
2s2p
3s3p
4s3d4p
5s4d5p
6s4f5d6p
7s5f6d7p

Done.

Number = Energy Level = Shell
SubShell = Letter (s, p, d ,f)
Orbitals are inside the SubShell:
s =1 orbital
p = 3 orbitals
d = 5 orbitals
f = 7 orbitals
Each orbital has 2 electrons. With opposite spin.

A copy of the device can be found in my comment here:
https://www.youtube.com/watch?v=uvfW-q1YR_4