You will find options for creating custom cardio or weight exercises under Exercise > My Exercises.
Select + Add a Custom Cardio, and type in a Name, the Workout Time, and the Calories Burned. When you're done, select Save.
You can select the exercise from My Exercises to log it or add it to your Favorites.
Select + Add a Custom Weights, and type in a Name, choose the type of Equipment, and the Muscles worked. When you're done, select Save.
When you open an existing Custom Weights entry to log it, you'll first enter the Reps and Time (there's a stopwatch timer available), and, optionally, you can add both the Weight and Notes.
When you select the green bar to log it, you'll see the data (Total Weight, Average Weight, Total Reps, Average Reps, and Total Sets) for the logged set in a list below.
You can select the ⋮ button in the log to delete the set if you've made a mistake.
In the pictures below, the History tab shows the historical entries for this custom exercise. The Stats tab graphs the data over time.
The ⋮ button at top of the custom weights exercise allows you to save it to your Favorites.