To use ad sequencing in a RF campaign, you need to create a Reach and Frequency campaign first by going to the campaign creation page and choosing Reach and Frequency as your Campaign Type.
After choosing the campaign type, you should fill in the rest of the campaign information in the first step and then go to Ad sets.
In Ad sets step, after defining your audiences you need to click on the green "Reserve" button. A pop-up will open where you can decide the period of time it'll be reserved for.
After clicking on the Creatives button and going to the next step, you need to set your creatives like you'd do in a non-RF campaign and continue to the Ads step. In the last step, you can set up ad sequencing by clicking on the little icon next to the ad set name.
To add sequencing, you need to click on the "+" sign in the pop-up that appears.
To finish and create the campaign, simply click on the Apply button and then click on "Create Campaign".