Check out this great guide to get unlocked, recovery and rooted:
http://forum.xda-developers.com/desire-510/general/guide-how-to-unlock-install-recovery-t2988292
If you bought your phone in the United States use 32-bit, if you bought it in Europe/Australia use 64-bit
****Make a backup of everything on your sd card****
If you have an sd card no bigger than 32GB, flash the kernel found here:
http://forum.xda-developers.com/desire-510/development/kernel-rhinstone-a11-t3013989
Use
Aparted to delete the card
Create one FAT32 partition (whatever size you want) and one ext4 partition (whatever size you want) or more partitions if you like at whatever size you want
Format all of your partitions
Use
Link2SD to link your apps and free up space on internal storage
****Make a backup of everything on your sd card****
If you have a 64GB sd card, flash the kernel found here:
http://androidforums.com/threads/virgin-sprint-kernel-desire-510-write-protection-removed.885145/
Use
Aparted to delete the card
Create one 32GB FAT32 partition and one 32GB ext4 partition (or more if you choose at whatever size you want, but I believe FAT32 is limited to 32GB size partition max)
[I am currently using one 32GB FAT32 partition and one 32GB ext4 partition and it is working great]
Format all of your partitions
Use
Link2SD to link your apps and free up space on internal storage
If you have an sd card that is bigger than 64GB, hopefully someone with more knowledge can help you out with that
Good luck and enjoy your extra space