The photos below show a an HTUD21 sensor rather than the Si7021 but the soldering procedure is identical!
Prepare the header strip:
Cut the strip to length if necessary. It will be easier to solder if you insert it into a breadboard - long pins down
Add the breakout board:
Place the breakout board over the pins so that the short pins poke through the breakout pads
Be sure to solder all pins for reliable electrical contact.
(For tips on soldering, be sure to check out our Guide to Excellent Soldering).