The greatest challenge in SFM image creation is finding very specific items.
That water bottle? Hi-Poly Spheres.

MODELS USED:
Undyne: https://steamcommunity.com/sharedfiles/filedetails/?id=770210346
http://sfmlab.com/item/2655/
Alphys: https://steamcommunity.com/sharedfiles/filedetails/?id=744916268
Papyrus: https://steamcommunity.com/sharedfiles/filedetails/?id=809966203
Sans: https://steamcommunity.com/sharedfiles/filedetails/?id=686820984
Toriel: https://steamcommunity.com/sharedfiles/filedetails/?id=607303223
Frisk: https://steamcommunity.com/sharedfiles/filedetails/?id=582496898
Monster Kid: https://steamcommunity.com/sharedfiles/filedetails/?id=784805186
Adam Kid as That Annoying Kid Down the Street: https://steamcommunity.com/sharedfiles/filedetails/?id=544724659

Hi-Poly Sphere: https://www.deviantart.com/pestilad/art/Kayakazan-s-Hi-poly-Sphere-Reupload-794263805
House: https://steamcommunity.com/sharedfiles/filedetails/?id=1443991897
Hotdog: https://steamcommunity.com/sharedfiles/filedetails/?id=504007098
Watermelon: https://steamcommunity.com/sharedfiles/filedetails/?id=1209990384
Grass: https://steamcommunity.com/sharedfiles/filedetails/?id=588558315
https://steamcommunity.com/sharedfiles/filedetails/?id=1679297510
Picnic table: https://steamcommunity.com/sharedfiles/filedetails/?id=450376169
Barbecue: https://steamcommunity.com/sharedfiles/filedetails/?id=450376169&searchtext=bbq
Fence: https://steamcommunity.com/sharedfiles/filedetails/?id=450913719
Cylinder (used for Frisk's leg) https://steamcommunity.com/sharedfiles/filedetails/?id=895978257
Skydome (I think it's this one): https://steamcommunity.com/sharedfiles/filedetails/?id=266679748
Grass Flatvoid (hidden under everything): https://steamcommunity.com/sharedfiles/filedetails/?id=230712305
Spatula: https://steamcommunity.com/sharedfiles/filedetails/?id=497762514

ASSET USED:
Lightwarptexture (whitecube): https://steamcommunity.com/sharedfiles/filedetails/?id=363777336

IMAGE EDITING:
Paint.NET: https://www.getpaint.net/

BONUS: scrapped idea
https://www.mediafire.com/file/yyqu1rccfpp5tyg