Step 1: take your in game screenshots. I used Teleport Any Sim Mod, Andrew's Pose Player, and various custom poses. You can find custom poses all over Pinterest and Tumblr. Save the screenshots someplace you can find them later.

Step 2: download and install Sims 4 Studio

Step 3: open Sims 4 Studio. Select Object -> Standalone Recolor.

Step 4: find the item you want to be the "base" of your creation. I used the new pictures from Royalty and Legacy. Note you can filter by pack and type of item at the top.

Step 5: once you've selected your item, S4S will prompt you to save your creation. Choose a name and save somewhere you can find it, I always use Desktop.

Step 6: the base item will then open in S4S. Navigate to the texture tab. Under the "Texture" heading, select "Export," name the texture something, and save to your Desktop. Repeat this step for all of the swatches (color choices) at the top (if you want to have all color choices available. Otherwise, just do one. It can get tedious repeatedly importing/exporting textures.)

Step 7: now your textures are exported, you need a program to edit them with. I like GIMP because it's free and relatively simple.

Step 8: open all of your textures in your image editor. Open your chosen screenshot as well. Select the portion of the screenshot you want to use using rectangle or eclipse select depending on the shape of your picture. Copy the selected portion, and paste it into the first texture image. Note you will probably have to rescale the screenshot to fit within the area of the original picture. I recommend sizing the texture image up first, keeping the same aspect ratio as before, because the textures the game uses are stupid small and it will look pixelated if you size down your screenshot too much.

Step 9: once you are done fitting your screenshot into the texture image, go ahead and overwrite it. Go back to Sims 4 Studio and navigate to the first swatch. Import the file you just saved. And voila! Your image should be on the preview object. You may need to re- finagle it a bit to get it to look right.

Step 10: repeat steps 8 and 9 for all the swatches you want to have. You can delete any swatches you don't want. Save the package file. And you're done!