Extraction treatment with aligners is possible and can be effective, but be careful! During the first years using aligners I worked with G6 protocol but for the last 3 years I decided to create my own plan for extraction cases.

I carefully choose my attachment design based on the treatment plan. 
Is it a full anchorage case? Will I use Tads?

Some of the considerations when planning extraction cases include objectives in anchorage, root control, attachment design, staging, incisor root torque, curve of Spee and AP changes. It is important to control the roots of the canines, bicuspids and molars. Add significant attachments on canines, bicuspids and molars with maximum length from occlusal to gingival, or better yet, optimized designs.

In this case we decided to make the posteriores unmovable and apply for FROG protocol on the anteriors. 
Do you know why we call it FROG? 
Look at the staging panel! Looks like a Frog looking to you.

We decide to move canines 8 aligners and stop, then 8 aligners the anteriors and stop, again 8 aligners the canines. This approach gives you a lot of aligners but makes the movement really predictable.

Have you ever tried the Frog Protocol?