Only enter them into the wallet that will secure/spend the fund -- in this video I'm using Coconut wallet, it's an application to recycle an old phone into an airgap wallet: the phone is meant to stay in plane mode forever after that.
To be clear, it is definitely be unsafe to enter your seed phrase into a third party application, and you shouldn't do so: this method works for any wallet, and you only need to enter the words in the wallet that'll secure the funds.