To Learn the Alphabet backwards, you need to have a mnemonic.  The mnemonic for learning it forward is Twinkle Twinkle Little Star.  We can use that same song to sing it backwards.  Here is the grouping

ZYX

WV

UTS

RQP

ONM

LKJ

IHGF

EDCBA

Now I’ve said my  my Z to A

Think I’ll go outside and play

Hers how the phrases of the original map to the backwards

ABCD  -> ZYX

EFG->WV

HIJK->UTS

LMNOP->RQP

QRS->ONM

TUV->LKJ

WX->IHGF

YZ->EDCBA

Don’t be surprised that some of the phrases are longer or shorter than you sing in the orignial, They don’t even have to match the number of syllables.  In many cases, you either drop the last note of the phrase, or double up on nodes.   For instance, the note where you sing ‘D’ does not have an analogue in the backwards one, as it is really a grace note between the BC phrase and the EFG phrase.