As we all know water is incompressible. So if you fill the tyre with it first, it massively reduces the amount of air you need to get the tyre seated properly. Best done outdoors for, err… obvious reasons.
(1) Seat one bead by hand or with an inner tube (unseat other side to get the tube back out again).
(2) Lay wheel and tyre flat on the ground with the unseated bead upwards.
(3) Fill with water through the valve stem, as much as you can until it dribbles out everywhere (a sealant syringe is handy for this and you’ll need one anyway).
(4) Inflate with track pump / CO? canister / 9p cokebottle device to seat the other bead.
(5) Let water out.
(6) Add sealant, install valve core, re-inflate, done.
Worked like a charm, having spent an entire evening failing to get a new Larsen 2.35 seated on my Bonty back wheel using all the traditional tricks.