Create a web folder inside the module folder and create the css file inside the folder like
Vendor_Module/web/css.
After that link the css in the layout file of the current module it may be in the default.xml or name_index_index.xml file.
the following code will be pasted in the head area of the xml file
<head>
<css src="Vendor_Name::css/yourcss.css" rel="stylesheet" type="text/css"/>
</head>