Here's how I enhance my images with SD upscale and inpainting:

First I take an image I like. For this example I'll use this one:
https://files.catbox.moe/rwny67.png
I'll send its prompt to the img2img tab with the "Send to img2img" button, but before upscaling I want to fix some details first.
Using a simple image editor I crudely fix any messed up details like mangled hands or broken perspectives:
https://files.catbox.moe/2fk22h.png

I then run the SD upscale script a couple times with the following settings:
>30 steps
>sampler: Euler a, and DDIM if my original picture is "grainy" and I want to retain that.
>CFG: same as base image (don't know if it matters)
>seed:-1
>between .15 and .35 denoising, depending on how many rough details need to be ironed out. (Something with crude edits made in paint will need at least .2 or higher denoising)
>upscaler: usually "ESRGAN_4x", sometimes "4x-UltraSharp" (download it if you don't have it)
>tile overlap: default 64 (I've never messed with this setting tbh)

Once I'm satisfied with the result (https://files.catbox.moe/8snbto.png) I'll press the "Send to inpaint" button.

Here I inpaint a new face with the following settings:
(Don't forget to switch off the SD upscale script here)
>masked content: original
>inpaint at full resolution: true (important)
>Inpaint at full resolution padding, pixels: 32 (mess around with this setting if you get weird black or discolored edges)
>sampling steps: same as before
>sampler: same as before
>denoising between 0.2 and 0.5 depending on how little I want the new face to resemble the face that's already there. If your base image doesn't even have a face, just set it to 0.5
>seed:-1

And that's pretty much it. Here's the final result: https://files.catbox.moe/zhkkqk.png

You could keep inpainting new details here over and over. If it's not adding what you want, just manually add a 1000 years in paint version before you inpaint, or even just insert a png of the thing you want to add, and it'll work.