Cut out a heart shape about this size from cardboard or paper.
This will be your stencil.

Place the stencil on the inside of your dress, wherever you want the love heart to sit (don't place it too close to the neckline).

Trace the shape onto the fabric using a fabric pen/marker (a normal pen works fine too).

Carefully cut out the shape using scissors.

Cut out a square of lace (big enough to cover the hole you just made in your dress, plus a couple of inches around each side - these can be trimmed down later).

Place the square of lace over the love-heart-shaped hole on the inside of the dress.

Use either a needle and thread or a sewing machine to go all the way around the heart shape (see picture).

Trim the excess lace on the inside of the dress.

You are done! Enjoy your new dress!