I personally don't like to use bulky yarn unless it is for a small project. When I substitute a smaller weight yarn, I make a swatch with a smaller hook for that yarn, then add stitches to get the right gauge, count how many stitches you added and round up one or down one if it isn't an even number. I think I know which blanket you are referring to, is it worked in the round? If so, just add the stitches to the beginning and end. For example, if you had to add 6 extra stitches to get the right gauge, then add 3 stitches to the beginning and 3 stitches to the end, following the pattern as it is written. Lets say round 3 calls for 10 sc (just throwing a number out there), then you would actually be crocheting 3 sc at the beginning, then the 10 sc for the pattern, then the 3 sc at the end. If it is a long pattern, sometimes I write the extra stitches in for each row to help. Keep notes as you go, so the second one will be much easier. Since you already bought the yarn you could try this for a few rounds and see if it works for you. As long as you have the same number of extra stitches added to the beginning and the end of each row/rnd, and follow the pattern it should work. Just remember that when you count your stitches after each round it will be different than the number given in the pattern, that's why I usually write my extra stitches down at the beginning and end of each line in the instructions, then write my new stitch total off to the side. Hope this helps and good luck!