I am a complete noob.

I want to make my own custom nes label. Can someone please guide me? I have no idea how to do it.

First you learn how to use some sort of graphic drawing applications. Usually they boil down to 2 types "raster" and "vector". Raster is easier if you want to paint free from. Vector is better if you want exact shapes. The application you may already have is probably "raster", in that case remember that pixels and real world units like inches don't always match unless the DPI ratio is fixed at print time. I recommend 600 DPI, which means multiply the number of inches by 600 to get the number of pixels. The GNU Image Manipulation Program and Inkscape are 2 nice free drawing applications.

For what size you should draw things in, when I did a label I measured the total area of a NES label to be 55 by 98 mm (or about 2.175 by 3.858 in). Earlier, tepples made a label at 200dpi which results to 432 by 764 pixels.

I'll let others mabye answer for how to print the pictures you draw and what to print on.

For printing, the safest option is to take your finished design to a graphics bureau and have them print it on glossy sticker paper. If your art contains crop marks, they might be able to cut it for you.

If you have to cut it at home, a ruler and a craft knife will work fine. For the corners, a corner cutter with the correct diameter is the best option, but a nail clipper will do the trick if you're careful: first, cut at 45°, then 22.5° and 67.5°, and possibly more if it doesn't look round enough for you.

To apply the label, peel just a little bit of one corner and cut the peeled piece of paper off, so that only a small part of the label will adhere to the cart. Line up the label on the cart, avoiding contact between the sticky corner and the cart until the label is perfectly aligned. Once the stick corner is glued, it will keep the label in place while you slowly peel off the rest of the paper and glue the rest of the label.