AviSynth can edit out unwanted frames, allowing you to do simple editing like removing credits from a movie or more complex editing like removing commercials from TV captures. If you know what frames you want to keep, you can add the Trim filter from the Editing category and directly enter the beginning and end frame on the Parameters tab. You can also use the Preview window to interactively edit a clip. When you have the Preview window open you'll see a trackbar at the bottom with a yellow line indicating the location of the current frame. You can use the left and right arrow keys or the scroll wheel on your mouse to navigate to the first frame you want to keep. Pressing
Note: If you have multiple trim points and the audio goes out of sync after the first section, try editing the Trim line, changing each + to ++
AviSynth 2.5 works in YV12 colorspace by default, but most of the applications that open scripts will require the input to be YUY2. You can convert to the correct colorspace by using the ConvertToYUY2 filter from the Processing category. Make sure to set the interlaced parameter to true if your video is interlaced.
The crop editor
The Core filters Crop and CropBottom can be used to crop lines from the edges of the video. When you have either of these filters highlighted, the Parameters tab has an additional button at the bottom which opens the Crop Editor. With the Crop Editor you can use either your mouse's scroll wheel or your keyboard to remove lines from any side of your video. Moving your mouse pointer so it's above one of the edges of the video will highlight that side to be cropped. Scrolling up with your scroll wheel or pressing + on your keyboard will increase the crop, while scrolling down or pressing the - key will decrease it. Click the Keys button to see additional options. The bottom of the window also displays the width and height of the video with the current cropping as well as Mod numbers, which indicate what number the height and width are multiples of. When you close the Crop Editor, the parameters will be filled in for you.
Note: The AddBorders filter, which adds solid color lines to the sides, gives you the option to use the Crop Editor as well, but you can't interactively edit the borders. Instead you can use the Crop Editor to view the current height and width, exit the Crop Editor, and enter the desired parameters in the fields on the Parameters tab.
The resize editor
Resizing filters (from the Resize category) change the resolution of the video. When using a Core filter that resizes by a variable amount (like BilinearResize, BicubicResize, or LanczosResize) the Parameters tab has an additional button at the bottom which opens the Resize Editor. In the Resize Editor you can select the width and height you want and see what the Mod number is. The total number of pixels and aspect ratio for the selected resolution are also displayed at the bottom.
Note: If your final format is a standard format for TV display (ie DVD, SVCD, VCD, etc,...) the aspect ratio can be a misleading number. For example, 704x480 and 352x480 will display different aspect ratios, but when you play them in your DVD player the aspect ratios will be the same. For viewing on your TV make sure to use the correct resolution for the format you're encoding to.
Other useful filters for beginners
Besides loading and editing clips, you may want to process the video to make it look better. The UnDot and Convolution3D plugins add excellent filters for this purpose while being easy enough for anybody to use. Like the name suggests, Undot removes dots from the video. UnDot has no parameters - just add the filter from the Processing category and you're done. Convolution3D is a denoiser with presets for different levels of noise in the video - movieHQ, movieLQ, animeHQ, animeLQ, animeBQ, and vhsBQ. There are 2 different Convolution3D filters listed in the Noise category so make sure to select the one with only clip and preset parameters. I've also noticed that if you select movieHQ for the preset AVSEdit doesn't add it to the script. To get around this you can use movieLQ and manually edit the script to change the L to H.
Note: Unlike most AviSynth 2.5 plugins, Convolution3D works in YUY2 colorspace so you'll have to use ConvertToYUY2() before Convolution3D. There's a beta version that works in YV12 colorspace, but not all the features of the filter are implemented, and since you will probably need to output YUY2 anyway, it's best to use the normal release version.