Short answer — three reliable ways:

1. Use Shopify’s built‑in “Buy X get Y” discount (simple, no apps)

• What it does: create a discount that gives a product free (or discounted) when customers buy a qualifying product or spend a minimum. Good for promo codes or automatic discounts. Caveat: the “get” item must be in the cart already — Shopify’s admin discounts do not auto‑add the free product for the customer. (help.shopify.com)
• Quick steps (admin): Discounts → Create discount → Buy X get Y → choose Discount code or Automatic → set the “Customer buys” conditions and the “Customer gets” item → save. (help.shopify.com)
• Pros: no app cost, works across POS (with POS Pro) and online checkout per settings; Cons: customer must add the free SKU, limited UI/UX (no gift picker/progress bar).

2. Use a “gift with purchase” app (auto‑add, gift picker, progress bars)

• What it does: apps can auto‑add a gift SKU to the cart, show popups/progress bars, let customers pick a gift, and remove the gift if qualifiers change. Examples: CartBot, Fokkio, Gift Box, BOGO+ and many others on the Shopify App Store. These apps are the usual way to give a truly automatic free gift and better UX. (apps.shopify.com)
• How to implement: install an app → create a GWP (gift with purchase) rule (e.g., spend $50 or buy product X) → select the gift SKU(s) and auto‑add settings → test on desktop & mobile → publish. Most apps offer targeting, limits per customer, date ranges, and analytics.
• Pros: best customer experience (auto add, banners, gift pickers). Cons: monthly app fees, must test theme compatibility, and watch inventory for gifted SKUs.

3. (Shopify Plus) Use Scripts / Shopify Functions for custom checkout logic

• If you’re on Shopify Plus you can create Scripts to automatically apply gifts/discounts at checkout and do advanced cart logic. Note: Scripts require Shopify Plus and Shopify has announced a migration path to Shopify Functions (Scripts are being retired — see the Script Editor docs). If you’re on Plus, check whether to build a Script or a Function depending on timing and Shopify guidance. (help.shopify.com)
• Pros: fully custom, runs at checkout, no third‑party app. Cons: Plus only; Scripts are technical (Ruby) and Shopify is moving to Functions (so plan migration).

Best practices (quick checklist)

• Use a real SKU for the gift so inventory and reporting are accurate. (Mark it as “gift” on the title/variant.)
• Limit per customer and set start/end dates to prevent abuse.
• Show clear messaging on product pages, cart, and checkout (e.g., “FREE sample added at $50+ — see cart”).
• Add a progress bar or banner so customers know how close they are to unlocking the gift.
• Test the flow: add/remove qualifying items, mobile checkout, discount stacking, and shipping cost effects.
• Track results (AOV, conversion, cost of goods, return rate) to make sure the promo increases profit, not just orders.

Which method to pick

• No app / low complexity: use Buy X get Y discounts (but customers must add the gift SKU). (help.shopify.com)
• Best UX and automation: install a GWP/cart auto‑add app (CartBot, Fokkio, Gift Box, etc.). (apps.shopify.com)
• Full control at checkout (Plus stores): Scripts / Shopify Functions — but plan for migration from Scripts to Functions. (help.shopify.com)

If you want, I can:

• Generate exact step‑by‑step admin instructions for the Buy X get Y flow, or
• Compare 2–3 top GWP apps for features/pricing and recommend one, or
• Draft the cart/announcement text and progress bar copy you can drop into your theme.