How to create a directory on sda while operating through live usb (sdb)

Greetings

I am trying to install LMDE 3.0 and have formatted my hdd and created two partitions namely sda1 - linux swap , and sda2 - ext4 for the installation.
I wish to create a root directory on sda2 for the purpose of creating a mount point for the installation.

My question is : How to I operate on sda2 while booted from the live usb sdb. Here's my output of for sudo fdisk -l

Disk /dev/sda: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 95374F94-C280-4A70-AFD5-C8361A51D85E

Device Start End Sectors Size Type
/dev/sda1 2048 4196351 4194304 2G Linux swap
/dev/sda2 4196352 976773119 972576768 463.8G Linux filesystem

Disk /dev/sdb: 7.6 GiB, 8110768128 bytes, 15841344 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x56c87540

Device Boot Start End Sectors Size Id Type
/dev/sdb1 * 0 3256319 3256320 1.6G 17 Hidden HPFS/NTFS

Disk /dev/loop0: 1.4 GiB, 1531080704 bytes, 2990392 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Any relatable advice would be helpful.
Cheers
Ps. Merry Christmas to all of you out there.