See either of the below.
https://github.com/fruestueck/MMM-WeatherDependentClothes
or
https://github.com/Lavve/MMM-WeatherOutfit/tree/main

this gets all your css and script dependencies worked out

then you take the smallest section of the body that makes your content visible (div, table, canvas…)

and make document. crreateElement(type) js api calls to create each

because you so that do much, in one of my modules i created a little function

createEl : function (type, id, className, parent, value) {
		var el= document.createElement(type)
		if(id)
			el.id = id
		if(className)
			el.className = className
		if(parent)
			parent.appendChild(el)
		if(value) {
			var e = document.createTextNode(value)
			el.appendChild(e)
		}
		return el
	},

which lets you do all the things at once create it fill in class names, attach to parent (building the tree of elements), set a value

see the bottom of my birthday list module for how i used it

https://github.com/sdetweil/birthdaylist/blob/master/birthdaylist.js

use my sample module
https://github.com/sdetweil/SampleModule

and see the documentation on creating modules
https://docs.magicmirror.builders/development/introduction.html

there are lots of modules that use the openweather api,
including the default weather module. you can examine those for the specifics

a module creates a small section of web content. typically part of the body section of a web page

MagicMirror will call the function getDom() to get this content
if you want to update the content at a later time, you inform MagicMirror by calling updateDom(), and MagicMirror will call again at getDom()

everything else in the module is about getting the info needed to supply getDom() with the info it needs

a web browser (electron, chrome, firefox, edge…) cannot read files directly or access hardware, for security reasons.
so MagicMirror provides support for a helper (node_helper.js) that runs outside the browser, and functions to send a message to it and receive a response
the helper is optional

the latest browsers have built in fetch() so that one can request data from other web sites directly without needing the helper