[Lenovo K5 Note Questions & Answers] comprocks:
Don't do it, mate! I had tried this exact same thing... Placing the su binaries manually along with busybox.

It sprouts up the "TOOL DL IMAGE FAIL!" error.

However, if you still want to proceed... Here are the instructions, copied from the SuperSU original updater script.

# ----- "SYSTEM" INSTALL -----
#
# "System" install puts all the files needed in /system and does not need
# any boot image modifications. Default install method pre-Android-6.0
# (excluding Samsung-5.1).
#
# Even on Android-6.0+, the script attempts to detect if the current
# firmware is compatible with a system-only installation (see the
# "detect_systemless_required" function), and will prefer that
# (unless the SYSTEMLESS variable is set) if so. This will catch the
# case of several custom ROMs that users like to use custom boot images
# with - SuperSU will not need to patch these. It can also catch some
# locked bootloader cases that do allow security policy updates.
#
# To install SuperSU properly, aside from cleaning old versions and
# other superuser-type apps from the system, the following files need to
# be installed:
#
# API source target chmod chcon required
#
# 7-19 common/Superuser.apk /system/app/Superuser.apk 0644 u:object_r:system_file:s0 gui
# 20+ common/Superuser.apk /system/app/SuperSU/SuperSU.apk 0644 u:object_r:system_file:s0 gui
#
# 17+ common/install-recovery.sh /system/etc/install-recovery.sh 0755 *1 required
# 17+ /system/bin/install-recovery.sh (symlink to /system/etc/...) required
# *1: same as /system/bin/toolbox: u:object_r:system_file:s0 if API < 20, u:object_r:toolbox_exec:s0 if API >= 20
#
# 7+ ARCH/su *2 /system/xbin/su *3 u:object_r:system_file:s0 required
# 7+ /system/bin/.ext/.su *3 u:object_r:system_file:s0 gui
# 17+ /system/xbin/daemonsu 0755 u:object_r:system_file:s0 required
# 17-21 /system/xbin/sugote 0755 u:object_r:zygote_exec:s0 required
# *2: su.pie for 17+ x86(_32) only
# *3: 06755 if API < 18, 0755 if API >= 18
#
# 19+ ARCH/supolicy /system/xbin/supolicy 0755 u:object_r:system_file:s0 required
# 19+ ARCH/libsupol.so /system/lib(64)/libsupol.so 0644 u:object_r:system_file:s0 required
#
# 17-21 /system/bin/sh or mksh *4 /system/xbin/sugote-mksh 0755 u:object_r:system_file:s0 required
# *4: which one (or both) are available depends on API
#
# 21+ /system/bin/app_process32 *5 /system/bin/app_process32_original 0755 u:object_r:zygote_exec:s0 required
# 21+ /system/bin/app_process64 *5 /system/bin/app_process64_original 0755 u:object_r:zygote_exec:s0 required
# 21+ /system/bin/app_processXX *5 /system/bin/app_process_init 0755 u:object_r:system_file:s0 required
# 21+ /system/bin/app_process (symlink to /system/xbin/daemonsu) required
# 21+ *5 /system/bin/app_process32 (symlink to /system/xbin/daemonsu) required
# 21+ *5 /system/bin/app_process64 (symlink to /system/xbin/daemonsu) required
# *5: Only do this for the relevant bits. On a 64 bits system, leave the 32 bits files alone, or dynamic linker errors
# will prevent the system from fully working in subtle ways. The bits of the su binary must also match!
#
# 17+ common/99SuperSUDaemon *6 /system/etc/init.d/99SuperSUDaemon 0755 u:object_r:system_file:s0 optional
# *6: only place this file if /system/etc/init.d is present
#
# 17+ 'echo 1 >' or 'touch' *7 /system/etc/.installed_su_daemon 0644 u:object_r:system_file:s0 optional
# *7: the file just needs to exist or some recoveries will nag you. Even with it there, it may still happen.
#
# It may seem some files are installed multiple times needlessly, but
# it only seems that way. Installing files differently or symlinking
# instead of copying (unless specified) will lead to issues eventually.
#
# After installation, run '/system/xbin/su --install', which may need to
# perform some additional installation steps. Ideally, at one point,
# a lot of this script will be moved there.
#
# The included chattr(.pie) binaries are used to remove ext2's immutable
# flag on some files. This flag is no longer set by SuperSU's OTA
# survival since API level 18, so there is no need for the 64 bit versions.
# Note that chattr does not need to be installed to the system, it's just
# used by this script, and not supported by the busybox used in older
# recoveries.

Put the files given under "source" into the "target" directory in the decompiled system image. Marhmallow API Level is 23. So only copy files that satisfy the API level number given.

Use 'chmod' command on the copied files to give it the correct permissions. 'chmod' is not a Windows command, so you will have to use Cygwin terminal or a Linux-based OS. Do NOT copy/move the files after applying the permissions because they will lose the permissions you set if copied.

'chcon' command can only be run from an Android terminal for the permission groups given, so don't bother with those.

Copy busybox to /system/bin and set permissions through the command 'chmod 0770 busybox'

Once you are done, re-compile the system image and flash it!

Believe it or not, if you did something wrong, your device will boot up fine... but if you did everything correctly, your device will fail verification of the system image and fail to boot.

That's all there is to it. Hope I cleared it up. If you still have questions, feel free to ask.