Add a heading or column containing instructions for New Item in SharePoint List

You can add in JSON Formatting in Header:

{
    "elmType": "div",
    "style": {
        "padding": "16px",
        "background-color": "#fff4ce",
        "border-left": "4px solid #ffb900",
        "border-radius": "4px",
        "margin-bottom": "12px"
    },
    "children": [
        {
            "elmType": "div",
            "style": {
                "font-size": "18px",
                "font-weight": "600",
                "margin-bottom": "6px",
                "margin-right": "16px"
            },
            "txtContent": "Important Instructions"
        },
        {
            "elmType": "div",
            "txtContent": "• Review all fields carefully\n• Do not enter confidential data\n• Report issues to the admin team"
        }
    ]
}

It will look like this in UI:

There are a couple of approaches you could take. 

1) Power Apps Customised Form

You could have a Power Apps customised form and have popup dialogs (using the new confirm() function) with the instructions on the dialog. In the example below there is an information button next to each field which pops up the instruction dialog. From the List-Settings -> Form settings select customize with Power Apps.

2) Configure the Modern List Form

But probably an easier approach is to configure the list form body and include your instructions there. So click on the + Add new item button and in the top right click on the edit form icon and select configure layout. You then add JSON formatting into the header, body and/or footer areas. In the example shown below I've added the instructions and set which fields are to be displayed in which each section of the body area.

The JSON is formatted like this:

{
    "sections": [
        {
            "displayname": "Please enter the information in the fields below using the official codes & data",
            "fields": [
                "Title",
                "IATA",
                "AirportType",
                "Elevation",
                "ScheduledService"
            ]
        },
        {
            "displayname": "Location",
            "fields": [
                "Country",
                "ISOCountry",
                "ISORegion",
                "Continent",
                "ISOContinent",
                "Municipality"
            ]
        },
        {
            "displayname": "Coordinates",
            "fields": [
                "Latitude",
                "Longitude"
            ]
        },
        {
            "displayname": "Codes",
            "fields": [
                "Idno",
                "Ident",
                "GPS",
                "LocalCode",
                "IDNumber"
            ]
        }
    ]
}

The instructions will then be shown whenever a user adds a new item or edits an item in the list as shown below.

A video by Reza Dorrani about configuring list forms is at https://www.youtube.com/watch?v=H4HyWJ9Seps

Rob
Los Gallardos
Microsoft Power Automate Community Super User.
Principal Consultant, Power Platform, WSP Global (and classic 1967 Morris Traveller driver)