You MUST start with 'unplugged' activities. These are going to be activities that explore computational thinking but are not completed on the computer. KS1 curriculum for computer science states that children need to understand algorithms. Algorithms are a sequence of instructions. They need to create a debug simple programs and use logical reasoning to predict the behaviour of programs.
You need to make a table with 6 or 7 columns and start drafting how you're going to teach over 6 sessions with progression. There is no point teaching a one off lesson. For example:
1. Talk about computing - unplugged activities on instructions - watch a video on what an algorithm is for young children - make own algorithms (instructions). I would do a simple activity like getting ready for school or brushing teeth.
2. For year 1 I would then do a dance algorithm - use the options: clap jump swirl kick and step. Print these on cards and put the children into groups - they can sequence their own dance with different algorithms, then children can move around and read other groups algorithms to complete their dance.
3. Drama - a farmer has to get a fox, a chicken and a sack of corn across a river he has a boat which carries him and only one other thing. How can they do it? Get the children to physically do this in the hall. Followed by some shared 'coding' - put something on the board (fish), then write next to it up up left. Ask the children where they think the fish might go. Give the children whiteboards get them to write some directions for the fish. Then draw a circle and put the fish in a different place - then get the children to write an algorithm to get the fish in the 'pond' .
4. Debugging - get the children to debug some sort of algorithms - make it simple so put up up left but make the fish go down down left. What's wrong? How can we fix it? Then an activity about that.
5. Get the children to explore on Scratch.
6-7. Bring the children back from scratch with printed off instructions - model model model model how to create a simple program. Create and present.
There are lots more ideas here
https://www.barefootcomputing.org/primary-computing-resources some of which I am sure will be much better than my 2 minute planning. Don't take this sequence as being 100% correct and it may not fit your class. There are loads of websites out there.