When you quilt with straight lines, how do you guarantee your straight line? I understand when you do the "stitch in the ditch," but do you draw the line with a marker in other areas that don't have a line to follow?

Depending on what I am doing, I have many times marked a straight line with a line of masking tape. I don't sew over the tape, just pull it up right before you get there but it gives you a good line to follow. Another way is to mark with a chalk pencil or disappearing marker and try to sew on the line.

I use quilters tape, 1/4 in and it is green . You just place it beside the seam, and then you can follow it and do a straight line.
Or I use a pencil and draw straight lines.

I usually just mark with my chalk pencil. Or, I mark one line and use the guide that came with my walking foot.

I have also used a hera marker to mark my stitch lines. It just makes a crease in the fabric so there is nothing to "erase" after you stitch.

I found a great way to do straight line quilting. In the book One Line at a Time: 24 Geometric Machine-Quilting Designs Made Easy by Charlotte Warr Andersen, she uses removable paper paper backed labels as guides to quilt by. No marking the quilt top and the labels comer right off and are reusable. She uses the 8 1/2 by 11 labels. I had to order them online. In her book she has straight line patterns you can trace and print on the labels. The labels really makes it easy.

Quilting by hand or machine? Most machines have an arm that attaches near the top of the presser foot, that helps you follow the last line of quilting, however far apart. Guess you have to make sure the first line is straight, or copy all the mistakes all the way across!

I found a tutorial at Crazy Mom Quilts for straingt line quilting. She used painters tape for her lines. I tried it on a stacked coin quilt I made for my grandson and it worked like a charm. I used 1inch tape and next time I will use 2 inch tape. I was able to use the tape over several times.

I will draw the first line with a straight edge. Once that line is sewn, my walking foot has an attachment which is similar to a hawaiian canoe outrigger. The outrigger can be set at the desired distance and runs in the ditch of the first line of stitches. As long as the first row is straight, the others will be too.

i use painters tape if i have the feed dogs down and if they're up and i'm using a walking foot i use the quilting bar.

if i'm using my quilting frame i use the back on the machine throat and i have wood spacers i can tape to the back of the throat. its a technique i learned with the flynn frame.