Clean the existing thermal paste from CPU and bottom of the heatsink. You can use IPA and some Qtips for the procedure.

My unit came with an AMD Ryzen 5 3500U quad core CPU with 8 threads

After completion of thermal paste application, follow steps in reverse.

Before connecting the battery, make sure everything on the motherboard is connected. Before you connect the battery, you should have completed all the tasks (reinstall CPU heatsink, install memory and the shield and plug in any other cables you removed)