You need an swf file for the interface you want to edit. If you're editing vanilla files extract them with BAE.

You need to download JPEXS FFDec.

First of all, to keep scaling intact you need to calculate your width by 960. If you play 16:9 1080p, your width by 960 is 1700 (and you also don't need this guide) Use calculateaspectratio.com if math is too hard. Enter your aspect ratio, enter 960 as height, copy the width. Multiply the width you got by 15 and write down this number. Since my monitor is 21:9 my width by 960 is 2240. Multiplying by 15 I get 33600. It doesn't matter what resolution I actually play with, only the aspect ratio matters.

Open the .swf with FFDec. In this example I'm using ExamineMenu.swf from DEF_INV.

Click on Header, then click on edit in the bottom.
Enter your width here, again, keep the height intact. On my computer it looks like this : 0, 0, 33600, 14400.
Click save in the bottom.

Go to frames > frame > PlaceObject2. Click on [+] on matrix. Click on edit in the bottom.
Subtract the original width (it's 25600) from your width. In my case, 33600 - 25600 = 8000. Divide by 2. Add the result to translateX.
If it's zero, simply input the number. In this case it's zero, so I enter 4000.
Click on save in the bottom, then and click the big save icon in the header.
Repeat for every PlaceObject2.

Note: sometimes the positioning of the object is dynamic, so this change will have no effect. In that case however it's probably either stuck to the left, middle, or right of the screen, so it should work regardless. HudMenu.swf for example only requires the header change to be fully functional.

You're done. Do the same for every file you need to patch.