Just follow below steps on How to flash Cm13 rom on Nexus 5 before starting download the following required file files and for stability report and Bugs in this rom head towards XDA thread here.
#How to install Cyanogenmod 13 Rom in Nexus 5
1.Download CM13 Rom for Nexus 5 and the appropriate Gapps 6.0 package from link given below Copy it in your in your device.
2.Now place the Cyanogenmod 13 rom and gapps 6.0 zip file in sd card.
3.Now reboot to Recovery mode and flash Android marshmallow rom in your device.
4.Then flash CM13 android marshmallow rom and gapps from recovery in it.
5.Now Clear cache and Clear Data.
6.Reboot your device.Done! you just flash cyanogenmod 13 rom in your Nexus 5 .Enjoy Android Marshmallow 6.0.
see also:Download official CM13 for Nexus 5