Corner to corner crochet, often just called C2C crochet, is a way to make a solid piece by creating crochet squares or blocks working on diagonal instead of traditional horizontal rows or rounds. Worked in a solid colour, it makes a deeply textured fabric that looks like a textured, layered version of the classic granny square. Introducing multiple colours to your work will give you endless possibilities to create a colourful final piece using any square grid-based colour chart/graph.
Working on diagonal, your work should be started at one corner and worked towards the other corner, creating a block of four double crochet stitches, and then increasing with one block in every following row until the halfway point is reached. After that your garment needs to be decreased with one block in every row until it is got down to one block on the opposite corner.
Please watch a detailed YouTube video tutorial by Tonya Bush on Basics of Corner to Corner (C2C) Crochet.