Adjusting pattern size

[Grace Bryant]

How can I make an 18 inch pattern 9 inches?

[Granny Square]

What sort of pattern?  A toy?  If it calls for US#4 weight yarn (like Red Heart Super Saver), use a much thinner yarn (if you are in the US, Michaels has a nice, and inexpensive acrylic sock-weight yarn called Woolike for example) and a much smaller hook appropriate for that size yarn.

It may not get you to EXACTLY half, you will have to do some swatching to figure out what hook and yarn combo is going to get you exactly half the gauge sizewise.

About pattern gauge, if a pattern says '20 stitches across=4 inches", you will need to hit 40 stitches across 4" to hit half the gauge, meaning half the width of the original stitch size specified.