I think if you search for how it install multirom on Nexus 5 you'll find good instructions.

To help you, here is the essence I've put together for myself:

1. Install back the stock android 6.0.1 rom as MultiRom only works with Android version 6 or less. Instruction

2. unlock boot loader. Instruction

3. Install Root: https://nexus5.gadgethacks.com/how-t...imers-0150493/
Download for root (CF-Auto-Root): CF-Auto-Root.zip

4. Install kexec kernel: (reboot into recovery and flash)
- kernel: kernel_kexec_hammerhead_601.zip
- multirom recovery: multirom-20160207-v33-hammerhead.zip

5. Install MultiRom. Instruction
- install 1.apk (multirom with UbuntuTouch repo) with adb. Instruction
- Download apk: 1-MultiROMMgr_withUT.apk (This the same 1.apk the xda site mentions, I only renamed to a meaningful name)

6. reboot into Recovery and install Sailfish as secondary rom
Install SailFish OS (Skip the Step 1: Installing cyanogenmod part, go directly to Step 2: Install Sailfish OS!): SailFish OS
- boot into recovery: Advanced -> MultiROM -> Add ROM
- Choose "SailfishOS" as ROM type.
- Choose first CyanogenMod ZIP and then SailfishOS ZIP. Click install.

I hope this helps. Good luck! (Give me a shout if you need help.)