How to crochet a diagonal line in a pattern

[Ace]

Hi! I am wondering how to crochet a diagonal line through a square. How would I make the line solid and not look patchy? When I try and attempt to do it, it looks patchy and you can’t really see the line. 

[Granny Square]

Welcome to the 'ville!

It's going to look jagged, it's the nature of the beast.  Take a look at these filet doilies, they are all beautiful (well I think so) but still stair-steppy.  Even smaller format needlepoint and cross-stitch 'pictures' don't have smooth outlines.

Working in a smaller format makes it seem less jagged; example colorwork done in SC would look better than those 'corner to corner' blankets where 1 pixel is 3 DC. I've seen some SC blankets that are amazingly detailed portraits or scenery.  Here is one example, an amazing portrait which if you look close at it would be pixelated.

Depending on your objective, example if the reverse side isn't going to show, you could do a line in surface slip stitch, if you really meant 'line' and not a diagonal solid shape.  Example, this shows 'drawing' a snowflake on crochet fabric with this technique, the 5th paragraph of the article links to how to do the stitch itself; it's just a slip stitch made by poking thru SC 'fabric'  versus into the top of a stitch as you usually would.