The old answer is a chroot jail, the new answer is a Docker container or VM if Docker won’t cut it.
I’m lazy, so Virtualbox is my VM software of choice. I keep a machine with a fresh debian install and just Clone it to make throwaway VMs.
Keep in mind that malicious software on a VM might be isolated from the host in many ways, but if it’s allowed to communicate on your network then it can still be dangerous, especially if you have samba shares, or services you don’t expose to the outside internet with weak or default passwords. (Did you change the admin password on your router’s Web interface?)
Creating a VM with no network interfaces is “mostly safe”, but you hear about VM bust out exploits now and then.
In reality, gold standard is a separate physical computer with no network connections to anything but other untrusted physical computers, and no wireless adapters (Bluetooth or WiFi). This is an “air gapped” network, but if you’re dealing with shit that makes you want an air gap, either you already know more than you’re gonna learn on Lemmy, or you’re bout to get your door kicked in by men in black suits :D