Harai-goshi is a judo throw that requires good balance. That balance comes from placing your support foot in the right place.
See Olympic Silver Medalist Gemma Gibbons demonstrate this technique.
Focus on the way Gemma pulls the sleeve upwards and outwards, and gets forward motion to complete the throw.
You can make your entry more dynamic by spinning in. Allow uke to move past you, then pivot on your support leg and spin in quickly. See how to do this entry here.
The spin-in entry can be used for other techniques like hane-goshi and o-guruma.
Use Sasae-Tsuri-Komi-Ashi to set up for this judo throw. Get uke moving in the circular motion using the ashi-waza to break balance. Follow up with the main waza.
Or follow up a blocked Harai-Goshi with an Osoto-Gari. If uke’s blocks your attack, then use this block. Switch direction. Throw uke backwards with Osoto.
Here is an exercise you can use to practise this spinning entry. Use a crash pad to break uke’s fall.
Both judoka should stand sideways on to the crash pad. Uke (in the blue judogi) begins the movement. Uke moves quickly in a semi-circle around tori. Tori remains still.
Tori moves when uke has completed the semi-circle, spinning in quickly to catch uke up and throw.