Adding grain without making mud

Badly applied grain looks like a dirty window in front of your footage. Applied well, it becomes part of the picture and nobody notices it is there.

Blend it, do not add it

Set the grain layer to overlay or soft light rather than normal. That lets the image underneath decide how visible the grain is, which is how film behaves.

Match the size to the frame

Grain scanned from 35mm should look like 35mm. If you scale it up to cover a crop it turns into noise, so scale your footage instead.

Export at a high bitrate

Compression treats grain as detail to throw away. Give your export enough data, or all the care you took disappears in the upload.