Use very light dumbbells, not more than 1-2kg to start with.
This sequence will develop straight arm strength for planche.
Maintain a hollow body hold throughout. Create global tension, legs extended, feet together, squeeze the glutes tightly. Shoulders should never touch the floor, keep the scapula protracted and squeeze inward to engage the pecs.
10 sec straight arm hold - Palms face upward, lock the triceps and don't allow any bend in your elbows.
10 Pulses - Keep the arms straight, perform short upward pulses. Lead with the elbows straight, keep the shoulders raised, scapula protracted, and core engaged.
5 Flys - Peform flys by squeezing your pecs together, driving the dumbbells up and inward, so they touch in the middle.