How should an SEO proposal be prepared?

If you want to look for a plan or want to create a new plan then you can follow the below checklist.

1 Set Up Google Analytics
2 Set Up Google Search Console
3 Install an SEO Solution for Your Website
4 Set Up Uptime Monitoring
5 Set Up Keyword Rank Tracking
6 Check for About, Contact, Privacy Policy, and TOS Pages
7 Check for Pretty Permalinks
8 Check the Site is Mobile Friendly
9 Check the Site Loads Fast
10 Keep URLs Short
11 Check and Optimize Your 404 Page
12 Make it Easy to Share Your Content
13 Claim Your Brand on Multiple Social Networks
14 Use Heatmaps to See What Users Actually See
15 Use a Top-Performing Web Host
16 Check The Server is Located Close to Your Service Area
17 Use a CDN
18 Check for GZIP Compression
19 Check for HTTP/2
20 Use a Caching Solution
21 Minify JS/CSS Files
22 Resize Large Images at Upload
23 Compress and Optimize Images
24 Lazy Load Images & Iframes(YouTube embeds, etc)
25 Update Technology
26 Optimize Database
27 Switch to Https
28 Check for One Single www Version
29 Fix mixed content
30 Create a Sitemap & Add it to Google Search Console(GSC)
31 Create a robots.txt File
32 Check that URLs have redirected to / or the canonical is set to /
33 Check and Fix Broken Links
34 Check and Fix Redirect Chains
35 Fix Missing and Duplicate Meta Titles and Descriptions
36 Use Redirects Properly
37 Check for Index Coverage Issues
38 Check for Manual Penalties
39 Keep Pages at Maximum 3 Clicks from Reach
40 Use “noindex” and “nofollow” Tags Properly
41 Disable Irrelevant Content from Being Indexed
42 Do Keyword Research
43 Have a Solid SEO Content Strategy
44 Create Long-Form Content
45 Create 10x Content
46 Identify & Fix Keyword Cannibalization
47 Identify & Fix Duplicate Content
48 Create Content Hubs
49 “Feed” the Hubs with Supporting Content
50 Update Content on a Regular Basis
51 Add Content to Your Categories
52 Build Up E-A-T
53 Write for People, Not for Search Engines
54 Check the Target Keyword Matches User Intent
55 Have the target keyword in URL, Title, and Heading
56 Use Focus Keyword in 1st Paragraph
57 Use a Single H1 on Each Page
58 Set an Enticing, yet SEO-Oriented Meta Title
59 Write a Compelling Meta Description
60 Use Subheadings for Content Hierarchy
61 Format and Style the Content Properly
62 Link to Relevant Inner Pages(Internal Linking)
63 Link out to Relevant, Authoritative Websites
64 Never Link to a Page/Website with the Same Anchor the Current Page is Targeting
65 Name Your Images Properly
66 Set Images Alt Text
67 Check for Social Markup
68 Check Dwell Time and Optimize It
69 Strike for the Featured Snippet
70 Don’t Over-Optimize
71 Have a Solid Link Building Strategy
72 Build Foundational Links
73 Interlink Social Networks
74 Guest Post on Niche-Related Blogs
75 Ask Partners you’ve Worked with for a Link
76 Turn Brand Mentions into Links
77 Use Social Signals to Boost your Page Rankings
78 Schedule Social Posting
79 Claim your Google My Business Listing
80 Claim Your Yelp Listing
81 Claim your Bing Places for Business Listing
82 Optimize Listings
83 Get on General Relevant Directories
84 Get on Niche-specific Directories
85 Reply to Reviews
86 Reach Out for Reviews
87 Publish Google Posts
88 Check for Site-wide NAP Details
89 Have Consistent NAP Details Across the Site
90 Have Consistent NAP Details on Listings & Social Networks
91 Have a Content Strategy Targeting Local Topics
92 Claim Brand Mentions from Local Sites
93 Add Local Business Markup to your Homepage

If you are looking for an SEO strategy: here are the steps to create an SEO strategy in 2020:

1. Step #1: Create a List of Keywords
2. Step #2: Analyze Google’s First Page
3. Step #3: Create Something Different or Better
4. Step #4: Add a Hook
5. Step #5: Optimize For On-Page SEO
6. Step #6: Optimize For Search Intent
7. Step #7: Make Your Content Look Awesome
8. Step #8: Build Links to Your Page
9. Step #9: Improve and Update Your Content

But if you are actually looking for an SEO plan, You need to hire an EXPERT.