Mains Hill is a reasonably prominent lump and sits above the A72 between Galashiels and Clovenfords. There are actually three 'summit' cairns up here, so you can choose whichever you prefer - there are great views from each of them...;-)
A waymarked walking trail - the Torwoodlee Circular - circumnavigates the hill, though part of it runs along the road between Cloven and Bowland. Although this is a minor road, please be aware that some of the traffic can hurtle along it at a seriously alarming rate.
There's no direct route to the summit and you'll need to pick your own way up the grassy slopes. Quad bike tracks may be helpful. Access from Clovenfords is straightforward enough. From the Torwoodlee side, you could go via Torwoodlee Broch; well worth a look.
I've suggested a number of trailheads but parking is limited. Perhaps the easiest access is from Clovenfords. If you're unfamiliar with the area then you could start out for 700m or so along the Torwoodlee Circular trail heading east along the side of the hill until you get to the gate at the Reference Point. Turn left and take the path going northwest from here on the west side of the stone wall to pass through the gate at the top of the field. You're now on the open slopes and just need to head upwards...;-)
If you start from the Balnakiel trailhead then don't be intimidated by the imposing iron gates and 'Private Road' sign. In fact, this route gives access to an optional diversion along a waymarked grassy path to Torwoodlee Tower which is tucked away from view in the trees.
If you take a detour to the tower then return to the junction and take the track going north to pick up the Torwoodlee Circular trail further up the hill. In any event, you are courteously requested to avoid the immediate vicinity of Torwoodlee House itself; if you're heading in that direction, you've probably gone off-route anyway...;-(
Not all the paths are shown on OS maps, but there is good coverage on OpenStreetMap. Enjoy the walk!
**** congrats to Cloventrout - FTF! ****