Adopt the terminology. "Stage right" and "stage left" are terms that relate to the actor's left and right. Downstage is close to the audience and upstage is far away. Center stage is in the middle. Use these terms in any combination to specify Helena's placement so as to best convey her emotional state.
Bring power to Helena's first lines by having her cross the stage to speak to Hermia. In Act I Scene I, she's expressing jealousy over Demetrius' love for Hermia. Direct Helena to march across the stage and stop close in front of Hermia. This is confrontational body language. Entering on the same side of the stage as Lysander and Hermia does not have the same power.
Place Helena downstage when she confesses her plan to tell Demetrius about Hermia's plans at the end of Act I, Scene I. Downstage confessions draw the audience in to the intimacy of Helena's schemes. Block Helena's lines as spoken to Demetrius' back in Act II Scene I. This reinforces how denied she feels by his rebuffs. Put Helena only a few steps behind him to highlight her tenacious love for him. When she begs him to use her as a dog, have Helena go down on her knees. Helena is willing to subjugate herself to him, and this position makes her vulnerable. Her pain and victimization at Demetrius' hurtful words are better dramatized if she's low and he's towering over her.
Put Helena's back towards Lysander during their conversation in Act II Scene II to signify her rejection of his affections. Have Helena remain standing while Lysander drops to his knees to confess his love. Her position is contrary to how it was with Demetrius. Direct Helena to wander all over the stage to show her confusion at this situation. She's convinced that Lysander is making fun of her by flattering her and she has low self-esteem. Have her confess how ugly she thinks she is downstage. This will bring the audience into her insecurity.
Arrange for Helena's entrance to occur before Lysander's in Act III Scene II because she is still rejecting his advances. He should be chasing after her. Have her stop center stage when Demetrius -- directly in front of her -- wakes and confesses his love. Her position center stage shows her as trapped between two men she thinks are making fun of her. Her center stage position is also a good one for her dialogue when Hermia enters. Make Helena circle Hermia when she accuses her of not caring about their friendship and being a part of the mischief. Reverse the circling when Helena is accused by Hermia of manipulating Lysander. Helena's hasty exit must be on the opposite side of the stage from Hermia, because she's fleeing from her.
Have Helena stand directly next to Demetrius when she wakes up in Act IV Scene I. Demetrius loves her now and they are a couple. Helena should exit the stage at the same pace alongside Demetrius.