Terraform docs for installing third party providers.
Grab prebuilt binaries from the Releases page.
Once you have downloaded terraform-provider-k14s-binaries.tgz, install it for terraform to find it:
mkdir -p ~/.terraform.d/plugins
tar xzvf ~/Downloads/terraform-provider-k14s-binaries.tgz -C ~/.terraform.d/plugins/Depending on the resource you are planning to use (ytt, kbld, kapp), you will have to install those projects' binaries. See k14s.io for installation instructions.