Please follow the instructions provided with the other emulator for installing plugins in that emulator.

If there is no mention of plugins, the other emulator almost certainly does not support plugins, and you cannot use the Project64 Legacy plugins with it.

If it does support Project64 Legacy plugins, you may be able to run the other emulator in your Project64 Legacy folder, or set both emulators to share a single plugins folder.

It is normally just a case of copy-pasting the plugins in the right place.

Notes

1. Some of the Project64 Legacy plugins are designed to work with the Project64 RSP / Icepir8sLegacyRSP - the audio plugin will not produce any sound in other emulators, and the video plugin will not produce some particular forms of graphics... In Project64 the RSP is processing these things.
2. Some of the Project64 Legacy plugins use the Project64 ROM Database Settings - you may wish to also copy Project64.rds to the other emulator.
3. Remember that the Project64 Legacy plugins will be sharing the same registry key(s) regardless of where they are on your hard drive or how many times you use them, so configuring them in any emulator will configure them in all emulators, and uninstalling Project64 may remove their registry settings - beware!