When I teach paper piecing I call it quilting by numbers. The number sequence is so important to successful paper piecing. When I design a paper pieced pattern, I try to make the number 1 piece the biggest piece in the unit. It doesn't always work out that way though and I think that confuses people sometimes. They naturally want to start with the biggest piece and add the smaller pieces afterward. There are times when you can ignore the numbers a little bit but those times are not the norm. So my first piece of advice is to make sure you start at number 1.
The first piece of fabric always goes right side up on the blank side of the paper and should be cut larger than the space it needs to fill. It can be pinned or glued in place. I prefer flat head pins because it bothers me when I can't get all the paper out when the block is finished. I like the flat head pins best because I can lay a ruler on top of them and the ruler doesn't "seesaw".
The ruler I like to work with for most of the project is the Add-A-Quarter Ruler. When I first saw it, my thought was, "I don't need ANOTHER ruler!". But when I tried it, I knew I had to have it. It has a lip that measures a 1/4" so I don't have to worry about putting the 1/4" mark of my rotary ruler is the right place each time I have to trim.
After I have my first piece of fabric secured to the foundation, I turn the foundation printed side up so I can place a card on top of the number 1 segment lining up the edge of the card with the line between the number 1 and 2 segments. I fold just the paper back over the card right on that line. Next, I place my Add-A-Quarter Ruler on top of the foundation making sure the lip of the ruler catches on the fold of the paper. Now I can trim the first piece so it has a 1/4" seam allowance. The next piece of fabric is placed right sides together on top of the number 1 piece lining up the edge of the number 2 piece with the trimmed edge of number 1. I can pin this piece in place if I feel the need, but if I use a pin I make sure it isn't close to my sewing line. All of the lines drawn on the foundation are sewing lines so I make sure my pin doesn't touch the one I am getting ready to sew on. I lower my stitch length so I am getting about 15-20 stitches per inch. On my Bernina I set it at 1.5. On Babylock machines, where you have to choose between 1.4 and 1.6, I would choose 1.4.
The shorter stitch length serves a couple of purposes. The paper takes up some space so when it is removed you don't want your stitches to be loose. The shorter stitch length makes them a little tighter. Also, with more stitches per inch, the paper is more perforated and will be easier to remove later.
Now its time to sew! Turn the foundation over, printed side up, and sew on the line between the number 1 and 2 segments. Start stitching at least a 1/4" before the line starts and end at least a 1/4" after the line ends. It is impossible to sew too far in paper piecing because any extra stitches and fabric will be trimmed off before adding your next piece. This second piece of fabric needs to be pressed open and trimmed before adding the number 3 piece. If the sewing line for number 3 piece intersects the first sewing line, when you fold the paper back on the line the first 2 pieces of fabric will fold back with the paper. Simply grab the fabric and gently pull it away from the paper so the fabric will lie flat on the cutting mat and the foundation folds back neatly on the line. Use your Add-A-Quarter Ruler or rotary ruler to trim the exposed fabric to 1/4" and continue adding your pieces in numerical order.