Normally Nintendo’s products are fairly straight forward affairs with little to know explanation needed. However, when it comes to the Joy-Con Straps, you’d be forgiven if some problems came up.
Putting them on the Joy-Con is a snap. It doesn’t matter which Strap you use for either Joy-Con, since each Strap is reversible. All that matters is that you match up the correct side to the right Joy-Con. This is easy to do: just make sure that the minus and the plus match the corresponding Joy-Con. Then you slide them on from the top down and they will click into place.
Once this is done, you will want to lock the strap into place. This is done by pressing the gray lever up. Now you can use the Strap for motion games or if you just want to use the Joy-Con sideways to play with the bigger L & R buttons on the top.
Removing the Joy-Con Strap seems to be where people get a little confused. Unlike when you remove the Joy-Con from the Grip or the Switch console itself, you do NOT need to press the release button on the back of the Joy-Con. Instead, you simply pull the gray lock lever down to unlock the Strap. Then you simply slide the Strap up and off of the Joy-Con. If you leave the lock lever in the locked position, it can be very difficult to get the Strap to release.
That should do the trick! Once you’ve done this several times it should be very fast and easy to do. Happy gaming!
[Source: Nintendo Support]