the mix for your concrete mow strip would be best done with a morter mixer materials would be 1/2 bag 94 lb portland typ one cement, 4 five gal. buckets sand, 1 handfull fibermesh, water will depend on how wet your sand is but a good morter mix consistancy like you would use for cement block work would be good simply for The fact that you are choosing the pain staking and time consuming task of hand forming your landscape borders.Make sure that you vibrate the form in some manor to eliminate air pockets. Let it set over night and pull the formes the next day. Then aproximatly every three feet you need to cut controll joints all concrete cracks this will controll it from cracking randomly. Don't worry about cold weather heaving your border it will expand and contract with the subsoil. This meathod you are choosing to go will be slow and messy,and with without experience? your outcome will probably be a product your wife will want you to rip out. I suggest calling a professional company in your area that already provides this service. they got it down to a science and will be able to provide you with different styles, colors,stamp patterns even lighting installed in it. They will install your border amazingly fast and clean with a finnish you would never be able to achieve. Go to this website borderlinestamp.com this is the landscape curbing capital and they should be able to direct you to a company in your area that does this process and if there isn't one you should look into starting one it's a definate money maker trust me.