Find the model file you want to use (usually .safetensors).
It can come from anywhere (browser download, cloud drive, shared folder, etc.).

Open this folder in a second window:
/home/clouddock/stable-diffusion-webui/models/Stable-diffusion/

Drag-and-drop (or copy/paste) your model file into:
models/Stable-diffusion/

Open A1111, click the refresh icon next to the checkpoint dropdown (or use Reload UI). Then select your new checkpoint and generate.
Done — your custom checkpoint is now available in A1111.