No custom nodes needed!
This is a very simple workflow for merging SDXL models by blocks/layers.
You can load it, select 2 models, adjust the merge ratio, and you can start generating

As general info:
IN blocks: contain concepts/composition/etc.
OUT blocks: contain style/aesthetics
The further away the blocks are from the middle, the closer they are to pixel level

SDXL has 20 blocks while SD1.5 has 26 blocks! Don't confuse them since most guides might show 26 blocks or IN11/OUT11 like in the image below. The merging logic stays the same though.

Here is an image from this article that helped me understand different blocks:
Here is another great article by RestlessDiffusion giving a tutorial on block merging in A1111/Forge using the SuperMerger extension and showing the effects of different merge ratios: https://civarchive.com/articles/2370/model-merging-management-how-to-merge-stable-diffusion-models-to-fit-your-style