To add Paypal, you can follow these steps below:
Step 1: Go to Payment providers
Go to your Shopify admin, select Settings and then go for Payment providers.
Step 2: Click on PayPal Express Checkout
You will see the PayPal section show up, it’s your job to click on PayPal Express Checkout.
Step 3: Click Activate
In the PayPal section, choose the PayPal account type that you want and then click on Activate.
Step 4: Login your PayPal account
From here, you will have to enter the email address for your PayPal account, hit the click Next. Then also, enter the password for your PayPal account, press Log In to log in the PayPal account that you want to connect to your store.
Step 5: Click Grant Permission
On the PayPal permissions page, you will have to click on Grant Permission to get another confirmation of this action.
Step 6: Back to Shopify
There will be another PayPal confirmation page, hit a click at Back to Shopify.
Step 7: Set Payment Authorization
Under Payment Authorization, just select two options:
Step 8: Save
Finally, click Save and then just make a test transaction at your disposal in case you want to make sure that everything is set up properly.
For the detailed guide with illustrated images as well as all about Paypal on Shopify, you can read here.
Hope it helps!