Something that I find works well, depending on tyre and rim, is brushing the rim with soapy water, and sometimes rubbing the tyre bead with a bar of soap. This usually provides just enough 'lube' to get a tyre on. Don't put the sealant in before attempting to get the bead of the tyre on properly.
once the tyre is properly seated, let out the pressure and remove the valve core, then draw up the right amount of sealant into a stan's syringe, and inject it via the valve into the tyre, clean the inside of the valve, and replace the core. now inflate to desired pressure.