Download the latest firmware build for PicoCart64 v1 lite.
You can find the last releases over at GitHub Releases.
A stable release can be found here, however newer versions might be available through the link above.
Unzip the downloaded file.
Program your Raspberry Pi Pico with either picocart64-v1-ntsc.uf2 or picocart64-v1-pal.uf2 depending on your N64.
Consult the official documentation for how to program uf2 files.
On this page, press "Choose file", and select the N64 ROM you want to prepare. Note that is has to be in z64 format.
Keep "Compress" checked unless you have a reason for using an uncompressed ROM.
Press "Build UF2" and wait for a while until the process is done.
Program the UF2 file which should be in your Download folder by now.
Ensure to power-cycle the Raspberry Pi Pico before starting the N64.