The 99 class machine uses a 66 bobbin. I find those machines don't like real thick thread. I usually use a machine with
15 class bobbins for thick thread and large 18 needles. If you don't use the same thread on top and in the bobbin
you will have problems on most machines.
You need the book for the machine and must follow it exactly. The flat side of the needle goes to the right and you
thread left to right. Not like a previous post. The bobbin must also go in a certain way. Hence the need for the book.
You shouldn't need larger than a 14 needle for your project. New upholstery thread should work fine. But you will have to change your tension settings to get the upholstery thread to stitch right. It is much slicker thread than say quilting
thread.