There seem to be plenty of sites if you google. 

Just looking at a couple the consensus seems to be

1.   Take the record out of the cover -  lay it, in its sleeve, on top of the cover

2.   Enclose them in a plastic bag (this is to allow taping later on)

3.    Sandwich them with two pieces of cut out cardboad, a big bigger and tape the record onto the cardboard

4.  Sandwich again with bubble wrap

5.  Sandwich again with cardboard

A sample guide is https://www.youtube.com/watch?v=5wdSWTc3HVk

But I haven't tried this myself.   The only time I sent records I used a pizza box and bubble wrap and they both got broken.

If you can afford it, buy professional mailers  https://www.ebay.com.au/itm/Brown-Rigid-Envelope-12-Vinyl-Record-LP-33-rpm-Postal-Mailers-345-x-345m...