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If you want to do a smooth sanding, you need a tool like this. This sanding tool, which looks like a hand plane, will come in handy.

Making Video :
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PDF Download : Sanding Plane Dimensions
Did you see this ? Hand Plane
Did you see this ? Sanding Blocks
You can use it both to drill 90 degree holes and to drill holes in the depth you want. It also works for joining two woods together, you can see in the video.

After watching this video, you can download the free PDF document and make it easily. There are many 90 degree corner clamp videos on youtube. But this is a little differ