After cleaning out the recess around the spark plugs thoroughly, remove the outer plug. Stick a straw into the hole, rotate the engine slowly (ccw) while wiggling the straw so it doesn't bind on the piston as it rises in the bore. At this TDC, wiggle the rocker arms (side to side, as you're unlikely to feel the small valve clearance with the oil cushioning it), if they're loose that's the TDC you want. Rotate the engine another 360* (straw at full extension again) and wiggle the rocker arms again, they should not wiggle, this is the wrong TDC. Do this two or three times until you're sure when the rockers are tight and when they're loose. Adjust the valves when both rockers are loose. Exact TDC (mark in the window), isn't necessary, as the valves are closed for many degrees on both sides of TDC.
Once you feel it, you're set for life. You can use a pencil instead of a straw, just remember to wiggle it as the piston rises or it might bind and break.
Something flexible is safer, like a straw.