Get an old inlet and exhaust valve. With silver solder, braze a small carbide or high speed steel tool to each valve so that after sharpening its radius is about 1mm more than the valve. Remove all the valves from your head or another head and insert thes two valves. Fix them on the guide side with an elastic band so that they do not fall out. Get the first piston you want to cut the pockets in to TDC. Fit the head wit these 'tool valves' using a head gasket and say 4 head bolts. Nip the head bolts. If the block has dowels to locate the head, use them. Now gently turn the first valve by hand and find the position where it touches the piston and sticks out the guide the furthest. This is your pocket starting point. Mark the valve with a black marker and scriber as you are going to machine all the pistond to the same depth. Using a hand drill, and one with a depth stop will de good, spin the valve and machine approx 1mm deep. This depends on the lift of the cam/rocker assembly. Do the same with the other valve. Remove the head, assemble one pair of good valves, a weak valve spring, head gasket etc. Set the valve clearances and with prestic on the piston, with a bit of oil on it so that the valve will not stick, turn the engine over at least 4 times. Strip all and make sure yuo have at least 1mm piston to valve clearance.
Cut the pocket deaper or if OK repeat with the other pistons. Don't worry about aluminium shavings getting between the piston and the bores; you can blow or wipe the bores when you are finished.
Good luck.