3j

<C-v>: to engage Visual-Block mode; then we define the column selection by moving our cursor down several lines

x...

x key deletes that column, . repeats the deletion for the same range of text.

gv

RESELECT our last visual selection

r|

replace each character in the selection with a pipe character.

yyp

we do a quick line-wise yank-and-put to duplicate the top line.

Vr-

replace every character in that line with a dash character.

Tip23 Tip25