The classical trick is to move the beads of the tyre into the well of the rim down to the center of the valley, so that the apparent tyre diameter become larger than the rim diameter. Then the tyre can be pressed with both hands towards the point where the lever can be finally used to insert the bead into the rim.
However, this is almost impossible with Marathon Plus tyres, which have a very strong wire and a inflexible rubber: when one spot is moved into the well of the bead, others go off, so that the diameter is never increased sufficiently.
To solve this issue, I tied several knots around the tyre with strings, so that the tyre is pressed into the rim.
However, with a classical knot, it would not be possible to press the rubber sufficiently so that the tyre would fit into the well of the rim, because these Marathon tyres have very little flexibility. To I used a Tarbuck knot, which makes it easy to get the sufficient pressure. One knot every 3 or 4 spokes allows to get a constant pressure.
It is then quite easy to push both sides of the tyre towards the point where the remaining work can be done with the levers, but without excess force.