Step 2: Solution

activity_main.xml

This is how your code should look by now

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    
    <android.support.constraint.Guideline
        android:id="@+id/guideline_verticalHalf"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_percent="0.5"/>

    <android.support.constraint.Guideline
        android:id="@+id/guideline_horizontalHalf"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.5"/>

    <android.support.constraint.Guideline
        android:id="@+id/guideline_horizontal38"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.38" />

    <android.support.constraint.Guideline
        android:id="@+id/guideline_horizontal30"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.30" />

    <android.support.constraint.Guideline
        android:id="@+id/guideline_horizontal20"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.20" />

</android.support.constraint.ConstraintLayout>

A vertical guideline has a width of zero and the height of their ConstraintLayout parent. A horizontal one has a height of zero and the width of their ConstraintLayout parent.

It doesn't matter if we're going to set the XML values of the width and height to wrap_contentor match_parent