I decided to attempt making a cipher, and here is what I came up with:

TUVWXYZABCEFGHIJKMNOPQRS
CDEFGIJKLMNOPQRSTUVWYZAB
YZABCDEFGHJKLNOPQRSTUVWX
BCDEFGHIJKLMNOPQSUVWXYZA
FGHIJLMNOPRSTUVWXYZABCDE
JKLMNOPQSTUVWXYZACDEFGHI
IJKLMNOPQRSUVWXYABCDEFGH
JKLMNOQRSTUVWXYZACDEFGHI
VWXYZABCDEFGHJKLMNOPQRTU
EFGHIJKLMNOPQRSTUVWYABCD
PQRSTUWXYABCDEFGHIJKLMNO
XYZACDEFGHIJKLMOPQRSTUVW
HIJKLMNOQRSTUVWYZABCDEFG
DEFHIJKLMNPQRSTUVWXYZABC


You are looking for a 14-letter phrase that describes this cipher's method of decoding.

small hint:

14 lines, 14-letter word. Think about it.

• @PiGuy314 From the first string of letters: rot13(Qb lbh trg "xf"?) Oct 4, 2021 at 14:44
• On the right track, but sorry. Oct 4, 2021 at 18:56
• @PiGuy314 for your second hint, are you referring to line 4? Oct 4, 2021 at 19:02
• rot13 (Npghnyyl, ab. Lbhe nafjre jnf irel pybfr va grezf bs gur fuvsg. Ubjrire, vs gur yvar fgnegf jvgu "N", gurer vf ab fuvsg, naq fb ba. Lbh pbhyq cebonoyl svther bhg gur erfg bs gur fuvsgf sebz gurer.) Oct 4, 2021 at 19:06

There are some errors in the cipher, I believe.

But here's what I did:

As the missing letters in each line were always an even number of spaces apart, i.e., always had an odd number of letters between them, I took the midpoint letter (just an AVERAGE cipher!) and came up with the string "hpksnjwinyxhtk".

Trying deciphering that with the first letters of each line, which should represent the amount to shift each letter (à la Vigenère), i.e. with the keyword "TCYBGQIMVEPXHD", I got "onmrhtowsuikmh". Not promising.

Next I tried enciphering it with the same key and got "arittzeuicmean". This was close enough to arithmeticmean that I figured that was the answer. (Though also, technically, this is not a fourteen-letter word.)

 TUVWXYZABCEFGHIJKMNOPQRS  "d", "l"  avg: H + T = a
CDEFGIJKLMNOPQRSTUVWYZAB  "h", "x"  avg: P + C = r
YZABCDEFGHJKLNOPQRSTUVWX  "i", "m"  avg: K + Y = i
BCDEFGHIJKLMNOPQSUVWXYZA  "r", "t"  avg: S + B = t
GHIJLMNOPRSTUVWXYZABCDEF  "k", "q"  avg: N + G = T **  (N should be B)
QSTUVWXYZACDEFGHIJKLMNOP  "r", "b"  avg: J + Q = Z **  (J should be W)
IJKLMNOPQRSUVWXYABCDEFGH  "t", "z"  avg: W + I = e
MNOQRSTUVWXYZACDEFGHIJKL  "p", "b"  avg: I + M = U **  (I should be H)
VWXYZABCDEFGHJKLMNOPQRTU  "i", "s"  avg: N + V = i
EFGHIJKLMNOPQRSTUVWYABCD  "x", "z"  avg: Y + E = c
PQRSTUWXYABCDEFGHIJKLMNO  "v", "z"  avg: X + P = m
XYZACDEFGHIJKLMOPQRSTUVW  "b", "n"  avg: H + X = e
HIJKLMNOQRSTUVWYZABCDEFG  "p", "x"  avg: T + H = a
DEFHIJKLMNPQRSTUVWXYZABC  "g", "o"  avg: K + D = n

So, to fix the cipher, I believe, the 5th, 6th, and 8th lines should read, for example:

GHIJKLMNOPQRSTUVWXYZBDEF
QRSTVWXZABCDEFGHIJKLMNOP
and
MNOPQRSTUVWXYZABCDFGHIJL
• Congratulations on solving the cipher, even with the mistakes I accidentally left in. : ) Oct 5, 2021 at 15:43

This is an incomplete answer as I've run out of time and must admit defeat. However, I've checked quite a few paths that resulted in dead ends, and discovered quite a bit more than what I can stuff into a comment. As such, I'm posting it as a partial answer.

Note: I will not be updating this answer any further.

Each line is:

Representative of a Caesar cipher. Their respective shifts are; 7, 22, 2, 23, 19, 9, 16, 13, 5, 20, 9, 3, 17 and 21.

However:

That's not useful by itself as each line is the alphabet, minus two letters. The missing letters are:

1: D, L
2: H, X
3: I, M
4: T, V
5: D, Z
6: B, R
7: T, Z
8: B, N
9: I, S
10: X, Z
11: V, Z
12: B, N
13: P, X
14: E, O

Other things I've checked are:

The acrostic at each index.
The acrostic of both primary diagonals.
The acrostics of missing letters
The alternating acrostics of missing letters (e.g. LHMTZ...).
Taking the letter of the alphabet representing each shift number.
Running a Caesar cipher (all shifts) on all the aforementioned points.

Hopefully this helps someone in the community come to a complete and correct answer.