1. Assuming 15 mm plastic pipe, you need two off 1/2" x 15 compression male irons. (They may be called irons but they are made of brass). E.g.
Screwfix 69358. Make sure they come with, or get separately, copper olives.
2. You will also need 2 pipe inserts to go inside the plastic pipe. You should use inserts of the same make as the pipe, and, if the make is JG Speedfit, make sure you use the inserts which do NOT have O rings on them. You may have inserts from the old tap push fit coupler. Again, beware the Speedfit warning.
3. Put the inserts inside the plastic pipe (which must be cut with clean square ends, no ragged or jagged ends).
4. Secure the compression end of the irons to the plastic pipe, using copper olives. Tighten firm finger tight, then about another 1/2 to 3/4 turn with a spanner. Use a second spanner or grips to hold the central hexagon of the irons to stop them turning.
5. Screw the ends of the flexis to the 1/2" threaded end of the irons. Finger tight then 1/3 to 1/2 turn.
The above assumes you either already have, or are not going to fit, isolation valves.