Short answer
Export your products to a CSV, fill the SEO Title, SEO Description and Image Alt Text columns from templates, and import the file with Overwrite existing products ticked. The free Shopify SEO bulk editor on this site does the filling in your browser with a Google-style preview. Use templates as a baseline and write the best sellers by hand.
Free tools for this
What the fields do
The SEO title and meta description are what Google may show in search results. If you leave them empty Shopify builds them from the product title and description, which is often fine but plain. Image alt text describes each picture for people who cannot see it and for image search.
Length limits
Google shows roughly 60 characters of a title and about 155 of a description, so keep to those. Shopify allows 70 and 320. Put the important words first, because the end is the part that gets cut.
Try it now: Shopify SEO Bulk Editor → · SERP and Social Preview →
Write a good template
A simple title template is the product title, then the type, then the store: for example {title} | {type} | {store}. For the description write one clear sentence about the product and the reason to buy, with delivery or returns. Leave out filler and the same claim on every page.
What to write by hand
Templates are a baseline. Your best sellers, main collections and homepage should have titles and descriptions written for what buyers actually search. Spend your time there.
Alt text that helps
Describe what is in the picture in plain words, with the product name. Do not stuff keywords. A template like the product title plus the image number is a start, and key images deserve a real description.
Import safely
Keep your export as a backup, import a small batch first, then check the products in Shopify and in a search snippet preview. Then run the full import.