Property Drawer

VisualElement

using System.Collections;
using UnityEditor;
using UnityEditor.UIElements;
using UnityEngine;
using UnityEngine.UIElements;

[CustomPropertyDrawer (typeof (SelectableItemTypeAttribute))]
public class SelectableItemTypeDrawer : PropertyDrawer {
    public override VisualElement CreatePropertyGUI (SerializedProperty property) {
        // Create property container element.
        var container = new VisualElement ();

        // Create property fields.
        var amountField = new PropertyField (property.FindPropertyRelative ("amount"));
        var unitField = new PropertyField (property.FindPropertyRelative ("unit"));
        var nameField = new PropertyField (property.FindPropertyRelative ("name"), "Fancy Name");

        // Add fields to the container.
        container.Add (amountField);
        container.Add (unitField);
        container.Add (nameField);

        return container;
    }
}

OnGUI

using System.Collections;
using UnityEditor;
using UnityEditor.UIElements;
using UnityEngine;
using UnityEngine.UIElements;

[CustomPropertyDrawer (typeof (SelectableItemTypeAttribute))]
public class SelectableItemTypeDrawer : PropertyDrawer {// Draw the property inside the given rect
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        // Using BeginProperty / EndProperty on the parent property means that
        // prefab override logic works on the entire property.
        EditorGUI.BeginProperty(position, label, property);

        // Draw label
        position = EditorGUI.PrefixLabel(position, GUIUtility.GetControlID(FocusType.Passive), label);

        // Don't make child fields be indented
        var indent = EditorGUI.indentLevel;
        EditorGUI.indentLevel = 0;

        // Calculate rects
        var amountRect = new Rect(position.x, position.y, 30, position.height);
        var unitRect = new Rect(position.x + 35, position.y, 50, position.height);
        var nameRect = new Rect(position.x + 90, position.y, position.width - 90, position.height);

        // Draw fields - passs GUIContent.none to each so they are drawn without labels
        EditorGUI.PropertyField(amountRect, property.FindPropertyRelative("amount"), GUIContent.none);
        EditorGUI.PropertyField(unitRect, property.FindPropertyRelative("unit"), GUIContent.none);
        EditorGUI.PropertyField(nameRect, property.FindPropertyRelative("name"), GUIContent.none);

        // Set indent back to what it was
        EditorGUI.indentLevel = indent;

        EditorGUI.EndProperty();
    }
}