Andy caught our camera in a store and asked about the proper preparation for a flagstone walkway.
I always joke that patios and walkways are for the Southern US and decks are for the Northern US and Canada, simply because dealing with water and ice always complicates anything we try to do down on the ground. So the key to a good walkway, be it flagstone or interlocking bricks, is to have a lot of frost free material under the stones. That means well compacted and well drained material.
Dig 10 inches below the final level of the walkway. Lay down a landscaping cloth that will stop things from growing up from below, stop sand from filtering down, and allow water to drain through.
Then add 6 inches of 3/4 inch gravel, tamping it down every two inches to make sure it is firm and won't sink later. Then 2 inches of well tamped pea stone or road gravel. This is a mix of small stones and finer material right down to sand. That mix of sizes allows it to pack harder and become more stable than the larger gravel below. Then put on the stones, which are generally 2 inches thick.
Sand can be added in-between the stones or the space can be filled with soil and grass seed can be planted. An alternative to grass is "Steppables", a grass and flower mixture specifically designed to withstand being walked on. You can get Steppables at speciality garden centres.
I really don't like the idea of trying to put mortar between the flagstones, unless it is on top of a solid reinforced concrete base, simply because it will shift and crack far too easily in a Northern climate.