Goal: one request, thousands of personalized messages, fully attributable.
Step 1. Build the personalizations array: one entry per recipient with their to address and per-recipient dynamic_template_data or substitutions.
Step 2. Attach custom_args per personalization: your internal user id, campaign id, or segment. These ride along with the email and come back in the events, so you can join SendGrid events to your own tables.
Step 3. Keep each personalization's substitution data under the 10,000 byte limit; large per-recipient payloads fail the whole request.
Step 4. Chunk large audiences into multiple requests rather than one giant call; a single malformed personalization can fail the batch.
Step 5. Join events back on custom_args: delivered, open, and click events per user id give per-recipient funnels.
Verify: test to seed addresses with distinct data each, confirm each seed got the right content, and confirm custom_args appear in the events. Pitfall: custom_args get no substitution, so put literal values there.