off-grain fabric HELP

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Newbie sewer here... I recently bought some red twill fabric at Joann's with plans to make a little a-line skirt. Yesterday I went to get started, and found that the fabric is seriously off-grain. Like WAAAAAY off-grain. Does anyone have a tried-and-true method for getting fabric back on-grain? My sewing teacher once showed me the blocking method, but I don't have a large enough surface to tack the fabric down.

Also, what is the best way to check the grain BEFORE buying?