Make sure the bowhair stays flat and there is no bowhand pressure. To check that the 1st finger is not gripping the bow, also practise without the 1st finger on the bow.
Find a point of contact not too close to the bridge, where the strings are not so taut.
Catch the bottom string, keeping the upper arm at the middle string level. Then add in the top string, still keeping the arm at the level of the middle string.
Finally, when playing 3-note chords with repeated down-bows, add some collé (in the air, make a silent collé up-bow action).