Don't just manhandle it. Using the push pull method, you do put the bottom limb tip in the instep of your shoe. You don't exert any pressure against the ground. All the pressure should be sideways against the instep. Pull back on the grip area of the bow while you're pushing forward on the top limb, with your fingers pushing the top string loop into the string nock at the same time. People have put their eyes out trying to do this, so be careful! Keep your eyes well below the level of the limb tip, so if it springs loose, it won't poke you in the eye.
It is useful to have a bottom limb tip protector, in case it does get jabbed in the dirt.
If this doesn't work very well for you, you can get a stringer specifically made for longbows, or one of those stringers that has a rubber friction pad.