The trick to a trundle bed is treating it as two beds, not one. Separate the trundle from the main frame first and each piece becomes much easier to handle.
Pro tip: Photograph how the trundle rails engage the main frame before disassembly, trundle bed mechanisms vary more than standard bed frames.
Watch out: A trundle's low rolling frame has exposed metal rail edges that catch ankles and scratch floors, keep it wrapped until it is safely loaded.