The Loop to Loop Knot
The knot assumes that both lines have loops in them, in this case both lines have figure of 8 knots but one end could be a swivel or other fixed loop such as a moulded loop on a plastic leader.
Take the shorter length ( in this case show in red) and slide its loop over the other loop ( such as the main line shown in green) , then insert the shorter end through the longer ends loop
As you pull the shorter piece to tension the knot you should see it start to come together like so
Finally as you pull it tight it should look like the above, if it doesn’t you have made a mistake in the 1st step. Disconnect it and start again.
There you have it the loop to loop knot made easy