[Q] Switching between Sprint CDMA and GSM SIM cards and back again topic

I currently am using my nexus 5 (unlocked from Play Store) on sprint and I will be going out of the country for a couple weeks and want to use a local SIM card when I get there. I have a couple questions.

1) what do I need to do to switch from Sprint CDMA and LTE SIM to a foreign GSM SIM?

2) once I get back in the country what do I need to do to switch from the from the foreign GSM SIM back to my Sprint CDMA and LTE SIM