Step 1: HTML Structure

1. Semantic HTML: Utilize semantic HTML elements (<header>, <footer>, <nav>, <main>, etc.) for clear page structure.

2. Proper Nesting: Ensure correct nesting of HTML elements, avoiding errors.

3. Valid HTML: Validate HTML markup using tools like W3C Markup Validation Service for compliance.

Step 2: Metadata Optimization

1. Title Tags: Each page should have a unique, descriptive title tag (within 60 characters).

2. Meta Descriptions: Write unique meta descriptions (within 150-160 characters) summarizing page content and including relevant keywords.

3. Meta Keywords: Include relevant keywords in the meta keywords tag.

4. Open Graph Tags: Implement Open Graph meta tags to control content appearance on social media.

5. Twitter Cards: Implement Twitter Card meta tags for Twitter content appearance control.

Step 3: URL Structure

1. Descriptive URLs: Use keyword-rich, descriptive URLs avoiding unnecessary parameters or IDs.

2. Hyphens vs. Underscores: Prefer hyphens over underscores for word separation in URLs.

Step 4: Content Optimization

1. High-Quality Content: Create valuable, relevant content for your audience.

2. Keyword Optimization: Strategically incorporate relevant keywords into content, headings, and metadata.

3. Heading Tags: Use heading tags (<h1>, <h2>, <h3>, etc.) with appropriate keywords.

4. Image Optimization: Optimize images for size and include descriptive alt attributes.

5. Internal Linking: Connect related pages through internal linking for improved navigation and crawlability.

6. External Linking: Link to authoritative external sources for additional context and credibility.

Step 5: Technical SEO

1. XML Sitemap: Generate and submit an XML sitemap to search engines for effective crawling and indexing.

2. Robots.txt: Create and optimize a robots.txt file to control crawler access.

3. Canonical Tags: Use canonical tags to indicate preferred versions of duplicate or similar content.

4. Structured Data: Implement structured data markup (Schema.org) for additional content context.

5. Page Speed: Optimize site speed by minimizing render-blocking resources, optimizing images, etc.

6. Mobile-Friendliness: Ensure mobile-friendliness and responsiveness for better rankings.

Step 6: Monitoring and Maintenance

1. Google Search Console: Regularly monitor site performance and health using Google Search Console.

2. Analytics: Track organic search traffic, user behaviour, and conversions using tools like Google Analytics.

By following these steps and continually monitoring and improving your Next.js web application's SEO, you can enhance its visibility and drive more organic traffic to your site. Remember that SEO is an ongoing process, so stay updated with industry trends and algorithm changes to maintain your site's search engine rankings.