This assumes you've created a new Integration in Magento 2 by going to M2 Dashboard -> System -> Integrations. Create a new one, give it Resource Access to ALL and save. Then Authorise it, then it'll create new consumer_key,consumer_secret,access_token,token_secret values for you.
Afterwards :
1. Create account / login on Postman.com
2. Create a new Environment - name it "API Magento"
Set up environment variables for :
"magento_api_url" : https://website.com/index.php/rest
"admin_username" : admin user
"admin_password". : admin password
3. Next create a new POST request to obtain a new admin token - - Set Environment to "API Magento" (so it can read the vars)
URL : {{magento_api_url}}/all/V1/integration/admin/token?username={{admin_username}}&password={{admin_password}}
params should be:
username : {{admin_username}}
password : {{admin_password}}
Click Send and it should return a token value - you'll need to paste this into the headers in the next step.
4. Create a new GET request for the REST API call
URL : {{magento_api_url}}/V1/products/PRODUCT_SKU
Headers tab:
Content-Type : application/json
Authorization : Bearer <TOKEN FROM ABOVE>
Authorization tab:
Type : "OAuth 1.0"
set "Add authorization to" : Request headers
Fill in consumer_key,consumer_secret,access_token,token_secret etc. using values from your Magento 2 integration set-up.
Click "Send" and you should get the REST API response with product details in JSON format.
JR August 2021
Helpful links:
Amasty Postman Magento2 Guide