Short answer, yes, remove the letters as you use them.
It is a kind of transposition cipher when the message is reordered in some way.
I actually just reorder it so I can see the message. If it was take every 3rd letter, I just line it up 3 to a row and then read down the columns.
I S A F F D I U M A L I M Y T S O T U U H C W A C I T E L X S L J
I S A
F F D
I U M
A L I
M Y T
S O T
U U H
C W A
C I T
E L X
S L J
IF I AM SUCCESSFUL YOU WILL ADMIT THAT XJ
“Tmcky ymu kra Ymu, tekt fs truar tekl trua. Teara fs lm mla kifva wem fs Ymuar tekl Ymu.” ~ Dr. Seuss
I love deadlines. I like the whooshing sound they make as they fly by. ~ Douglas Adams
If you're not part of the solution, you're part of the precipitate. ~ Henry J. Tillman
Some people are lost in the shuffle. I'm shuffling along with the lost.