Sukui uke is a scooping movement that can be found in several Kata. Especially Kushanku features Sukui Uke and Bassai has it too (in older styles the last two moves are not Soto Uke but Sukui Uke).
The purpose is to get below the attack in order to scoop it upwards. Sukui Uke easily breaks the opponents posture. It is common to use Sukui Uke against kicking techniques, or to use it to (continuously) execute a throwing technique. From a lower position you could even scoop his supporting leg.
I hope you read it before: ISOLATE.
You HAVE TO isolate... It's the fastest way forward.
This method isolates Sukui Uke completely so you can focus on the basic movement timing it to catch his leg. Nothing else..
Practise this one first— then the application of Kushanku down below.
Sukui means scoop, which in case of application can mean that you catch Maegeri by bringing your arm underneath the kick. To execute this technique in one movement, you have to get quite close to your opponent. You can use Irimi to move to the right place and execute Sukui Uke. After Sukui Uke as I demonstrate it in this video, a takedown is the most logical next step.