Obviously Ymir's costume is exactly the same as Sasha's but swap out the white button up for the gray t-shirt and that's it, so I'll just copy and paste what I wrote for Sasha on here lmao. I love cosplaying Ymir (and Yumikuri!) though, they're essentially the only characters that I care about in SNK anymore haha.

I bought the survey corps jacket online, the white jeans at Kohls, I had the white undershirt, and the black boots at Goodwill. I made the straps out of brown pleather, I cut it up into strips and basically sewed like, around maybe 10-15 yards in length of straps. Getting the shape of the straps was pretty difficult, I made it so that they all stayed together by looping through my belt I wear around my waist. When it's all on, I think my waist belt loops through the straps like 10 times or something - but it works!! Bending over is not easy but what can I do.

The 3DMG was all made out of posterboard. Laboriously carved out the pieces with a knife and glued them all together with hot glue. The 'tank' on the top is a water bottle cut in half, with rolled up "poster paper" - basically thick paper - glued together (I probably wouldn't recommend this because it was pretty fragile). Spraypainted all of this silver and used black craft foam for the things that hold it all together.

The blades were also made of posterboard, cutting out the intricate shape of the handle was quite a task - I used this tutorial for them.
http://cosplayruinedmylife.tumblr.com/post/53624044593/basic-craft-foam-attack-on-titan-3dmg-blade