The T-shirt Quilt Update

Materials
Tee shirts (4 to 36 = number depending on size of quilt), Interfacing (fusible, woven type), woven cotton fabric for sashing and borders, fabric for backing (woven cotton, flannel, or polarfleece) and batting if you choose to use it.
Interfacing
There are many brands of interfacing. Look for a woven type or a nonwoven that is not stretchy (don't use a knit interfacing). You are using the interfacing to make the knit tee fabric non-stretchy for ease in sewing. Some interfacing needs to be pre-shrunk with water to avoid bubbles on the surface after fusing so read the directions from the manufacturer and test on a plain piece of tee shirt knit fabric.
Cotton Fabrics
You'll be using woven cotton (ie. regular quilting cotton) for the sashing and borders of your quilt. You want the stability of a woven cotton, not stretch from a knit. Pick a fabric that will frame your tee blocks nicely. You don't need to use a sashing, you can sew the blocks directly to each other.
Batting
You may want to use batting. A polyester batting with loft is fun if you are going to tie your tee shirt quilt. A cotton batting or blend is better for a machine quilted project. If you want a thin style quilt you could leave the batting out completely.
Backing
You may want to use tee shirts pieced on both sides if you have a lot of them. If not, a plain cotton backing used with batting is good. Or a flannel or polarguard backing is cozy and you can leave out the batting with these, too.
Preparation & Techniques

Decide Layout
Assuming you have all your tee shirts out and have studied them and possible layouts so you know your block sizes (see the section here on Blocks and Quilt Layouts to help plan, also look at the Gallery page to see sample quilts).
Washing & Cutting
Wash all tee shirts, if not previously worn you may want to wash them twice. Take the side seams apart (or if they were knit in the round you can slice one side and remove sleeves. Remember to save all the image areas if you t-shirt has small logos on sleeves, etc.
Fusing
Pre-shrink the fusible if needed. Cut the fusible interfacing about 2 inches larger than the size of your unsewn blocks. You may want to use a 1/2 inch seam allowance for your tee shirt blocks rather than the traditional 1/4" of quilting to make sturdier seams. Following fusible manufacturer's advice, fuse the interfacing to the tee shirt sections you plan to use.
Cutting the fused tee shirts
If all your blocks are to be the same size, you will find it convenient to make a template from cardboard, mat board or plastic template material. If you are a quilter with a rotary cutter, this is definitely the tool to use. Cut using a long acrylic rotary ruler and a mat. If using scissors, trace your template and cut. Cutting the sashing and borders: remember to leave 1/2 inch seam allowance if you choose to have extra. Thus a 3 inch sashing finished sewn size will mean you should cut a 4 inch wide strip.
Arranging blocks
If all your blocks are one size you can go directly to arranging their order. But, if you are using some smaller units of tee shirt images to make larger blocks (in a 4 patch block for instance) sew those first. Then clear off a place on the floor, a bed top or hang a big flannel sheet or batting on the wall to arrange your blocks.
Sewing
Sew blocks in rows, either horizontal or vertical inserting sashing pieces (if desired) as you go. Then assemble rows and add the borders.
Finishing
Layer top with backing and batting (or leave batting out if you choose) and quilt. You may want to finish this tee shirt quilt in the "reversed bag method" which is especially easy for a tied quilt. Layer the parts this way: put the top of the quilt right side UP and tape to the floor or tables with masking tape evenly around all edges, then put the backing fabric right side DOWN against the top (ie. the two right sides are together), then place the batting on top (if desired to have batting). Pin or baste to hold the edges together. Sew around 3 full sides and 2/3 of the 4th side leaving an opening with a 1/2 inch seam. Trim the batting close to the seam, but don't trim the fabrics and turn the whole quilt right side out. Hand sew the 1/3 opening on the 4th side. Tie closely every 4-5 inches or quilt the quilt