I put a short stack of sticky notes on the line I want to sew on and butt the fabric up against it. Got that tip from Lynette Jennings How to Quilt book. Works really well.

I have a 1/4" foot that helps but still weave off at the beginning and end. These are all great ideas.

I find it hard when sewing along the diagonal line of a square to keep the sewing line straight to the end. I tend to end with a curve ! The end of the corner is delicate so ripping out sometimes takes fabric as well as thread..

Sometimes the fabric is up to tight against the blade when we start, then as we finish, we tend to relax,and the fabric comes away from the blade. measure the seam allowance at the start and finish, and see if it is smaller at the start and bigger at the end. this is what was happening to me when i first started.

there are alot of aids out there you can use to help keep the fabric straight. free ones,(like mole skin and post it notes) and ones you pay for..(ther is a magnetic guide,a screw in guide, a plactic one too) use what works for you.
i also painted the front ot the blade tip white when i first started..so i could see it easier..(i used white out..lol)

i watch the guide or the edge of the foot/fabric, never the needle.

I use Dr Scholls moleskin....which is basically adhesive backed felt but with much better adhesive than the usual adhesive backed felt. Stack 2-3 pieces and stick them together then cut a straight piece about 1" x 3" and stick it exactly 1/4" away from your needle. Snuggle your pieces up against the edge of the moleskin and sew. Guarantees good seams every time!

I have painter's tape in my sewing drawer for that exact purpose. It works also. I run it all the way across the machine in case I have a rectangle square (got that one?) or a strip and then I am better able to keep track of my fabric. Like I am going to lose a piece of fabric on the sewing machine. I have been known to do that too! Edie

I use painter's tape on the diagonal line of the square to line up my stitches. Same thing for quilting lines. Follow that tape!

Keep your eye on the foot not the needle. That has helped me. I always used to sew crooked too.

Thank you all so much. These are all great suggestions and easy to do. I like the idea of making sure I'm sitting straight at the machine. The painter's tape is a good idea, plus marking the line. I have a Sewline marker, which is fabulous for things like this. In fact, all of the suggestions are useful. I could try every single one of them, and then I can see which one works best for me. Thank you again.

A couple of things helped me new glasses with antiglare and a 1/4 in foot