To do the trade, you are require to have a Nintendo Online Subscription.
Step 1: Press Y (To make the Y-COMM menu pop up)
Step 2: Select Link Trade
Step 3: Select Set Link Code and you'll have a screen pop up to select a 4-Digit code. (I'll send this specific Pokemon code before we trade).
Step 4: Once found, your Pokemon box will be shown and you can select any Pokemon you wish to offer for trade (any scrap Pokemon you catch in the wild will do) You can also see what Pokemon I will be sending you at the top of the screen.
After we've selected the Pokemon, it will go to another screen where we can both accept the trade (you can check the summary of the Pokemon here)
Step 5: Select Trade It it to offer your Pokemon and the trade will go through. We then repeat depending on the amount of Pokemon you have purchased.