Thank you! *Running off to learn how to stitch in the ditch*

Are you going to machine quilt it or hand quilt it?

Machine quilt...a row at a time. I think if I tried to hand quilt, it would take me years to finish. Wait...does anyone hand and machine quilt? Like on the same quilt? With some parts machine quilted and other parts hand quilted? Is that a thing?

When you quilt, start in the middle and work your way out. Otherwise you'll get unwanted puckering. How did you baste it--with pins? spray baste? thread basting? If with pins, be sure to place each one about a hand width apart, and remove them carefully, don't hit them. (Very bad for machines--more than a broken needle, a little piece can get down in the machine and damage delicate part. Worst of all, a pin could break and fly into your eye. Or close to it. Ask me how I know. Glad I was wearing glasses!)

Have you quilted anything before? If not,I'd suggest making a practice sandwich with maybe a big four-patch on top so you can practice SITD. For my first quilt, I did a big X through each block and a wavy line through the sashing .Have fun!

Welcome to the board from Oklahoma! Wonderful job so far....I vote for the gray color thread also.

Thank you for all the great advice! This is my very first time quilting anything after years of admiring quilts from afar. I spray basted my first row, but thinking I might use pins on the others. The spray made things pretty difficult to line up, and the fabric kept flopping over and sticking to itself, so I'll probably pin the next row and see if that's easier. Good tip on pin removal. I definitely don't want to lose an eye, especially while quilting. Don't want to get blood on my fabric. :p

Making a practice sandwich is a really good idea. I have a tendency to jump in way to fast, so it's probably a good idea to do a few practice runs.

If you go with a 50wt cotton thread, it will be less shiny & will blend right in with the background fabric. Embroidery threads are typically rayon or polyester & those have a sheen to them that draws more attention to itself. Gutermann is a nice inexpensive 100% cotton thread. If you really want a top-notch thread, Aurifil is sold online & in select quilt shops (though usually only in a few dozen colors, not the 260+ colors they sell through online retailers). I'd go with their 50wt. It's a 2-ply and if you get it to match the grey, it really will barely be noticeable even from a foot away.

Be prepared for it to look very different after quilting. Not bad....but different. I remember my first quilt. But he will LOVE it if he has posters all over the walls. Keep going.....you are almost done.

Funny you mention cotton. I went through my stash of embroidery thread and the grays I have are too dark. So I ordered some 50wt cotton. I found a pack of 4, all different shades of light to medium gray. And I was hoping that cotton would be okay.

One more thing...
Part of me is reluctant to suggest this, but as a beginning quilter, I think this might be a good time to break my general rule of quilting every 2". This isn't a quilt that is going to be washed every week, neither is it likely to become a family heirloom. It is a wonderful, loving gift to your nephew from you. So here's what I'd suggest for the quilting (make sure you either have a Dual Feed machine or else you'll need to install a Walking Foot to do the quilting). I'm not sure what kind of batting you used. Some battings need to be quilted every 2" or else they bunch up inside the quilt when washed. Other ones, like Warm & Natural can be quilted 10" apart or more without issue. Assuming you have a batting that can be quilted farther apart, here's what I'd recommend:

Note: The orange lines are so they show up in my illustration. I would quilt in that medium grey of your background fabric.
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Trying to quilt inside the TWD fabric gets a lot trickier unless you just quilt right over the print & don't worry about it (which isn't as horrible as it sounds). Over time, with many more hours of practice, you may decide to try your hand at Free Motion Quilting which would allow you to easily change direction within the rectangle to outline all the little shapes & even do decorative designs around them. You need to Stitch in the Ditch first for those designs anyways, so this will be a great practice piece for you to learn more about quilting. Just go slow (set your machine on the slowest setting if you have a speed dial). Stitch length should be 3mm or 9 stitches per inch is a good length for SID. Or you could make it a bit shorter (as short as 2.5mm or 10 stitches per inch). The walking foot, if you need one, is pretty clunky & it can be hard to tell if it is up or down. If you don't see your fabric moving when you press the speed pedal, odds are the walking foot (presser foot) is up. Same thing with nests of thread.

That said, it really isn't terribly difficult to learn to quilt with a walking foot (or dual feed machine) if you already know how to use your sewing machine for basic sewing. Making a practice piece (at least 12x12, preferably Fat Quarter size which is 18x22" or even a 1/2 yard) does help, though. It will allow you to get your machine settings just right for your quilting project. You usually need to lower the top tension to somewhere between 1-3, depending on your machine. You want the same thread in the top & bobbin. And make sure you use a fresh needle. Universal needles do not work great for quilting. I love Microtex needles. For a 50wt, 2 ply thread, a 70/10 Microtex (aka, Sharp) needle works well. For a 50wt, 3 ply thread, a 80/12 Microtex needle works nicely. You can also use a Top Stitch needle in size 80 or 90. Or a Quilting needle. Needles should last 8 hours of quilting time if undamaged.

BTW -- love your design!!! Your nephew is one lucky guy!