The frame I will be using for this tutorial (which is the frame I use on 99.9% of my stamps) I found from the above linked post, which I believe is from a creator that OP lost track of. Its inner dimensions are 94x50 pixels and its outer dimensions are 99x56 pixels. Here it is!

Find your material! - I recommend using websites like Tumblr and searching with the “GIF” filter only on, or alternatives such as Giphy or Tenor. Your browser may let you directly save the .gif file; if not and you are noticing it restricts you to save it as a .webp file you can try an extension like “Save webp as PNG or JPEG” (for Firefox but I image other browsers have similar functions, but I really recommend you switch to Firefox). To use this you will right click on your source .gif like normal but instead of clicking on “Save image as…” click “Save webP as…” and then click “GIF”. You should be redirected to the website ezgif.com where we will actually be doing all of our editing! Here’s the .gif we’ll be working with.

Convert to GIF (optional) - if you used the extension from the above step you should already be ready to click the blue “Convert to GIF” button. If not, go ahead and open ezgif.com and click on “webP” and then “WebP to GIF”; then convert to a gif with the blue button.

Resize the GIF - now that we have a gif ready to edit, let’s make it the right size. The easiest method I have found is to change it directly to the frame’s inner dimensions, 94x50 pixels. [EDIT: Make sure in the aspect ratio drop drop menu you select "stretch to fit" and not "center and crop to fit" like I did in the photo example.] Click “resize” and then type [94] in for the width and [50] for the height. Next press the blue “resize image” button.

Add the frame - next click “overlay” then click the thin blue button that says “Extend canvas size(use if overlay exceeds GIF sizes)”. This will give us some extra room to add the frame onto the design. Next click “Browse…” and find the frame you have saved onto your device, then click the blue “Upload image” button.

After that it’s going to be misaligned, that’s normal! It will say you have the option to drag it into place, but don’t bother. That’s one of the reasons my old stamps look wack, it’s just harder to do. Instead type [44] in for the Left box and [22] in for the Right box. It took me a while to figure out these dimensions to be honest, and I’ve only tested it with this frame so I don't know if it works with others. Then click the blue “Generate image” button.

Crop the transparent edges - click on “crop”. You will have the option to check a box that says “trim transparent pixels around the image” however, I don’t recommend this as it tends to crop a few of the frame’s pixels with it sometimes. Next, set the Left position to [44] and the Right position to [22]. For the other dimensions we will use the outer dimensions of the frame which are 99x56 pixels, this will trim everything except the tiny spaces in between the stamp frame’s spikes. Type the width as [99] and the height as [56] and click the tiny blue button that says “set”. After that click the blue “Crop image” button.

Save and use! - all that's left is to click “save” and upload the graphic to your liking. (best seen on dark mode obviously)