Play up the comet’s contrast
A good way to capture a fainter comet can be to imaging them
If you know the area of sky that the comet should be found in, but can't find it with binoculars or a telescope, another method is to use a DSLR camera. You will need a telephoto or zoom lens and a tripod for this. Use the camera manually; having control over the ISO and exposure settings is important. Once you've set the camera up you can take several shots with different settings to see which is best.