Now that you've gone through all the effort to figure out "the plan," make sure you're writing it all down.
The general approach is to write out your workout (or workouts if you're doing splits) with the name of each exercise leaving room to write the sets and weight. Tracking this information is key! It will allow you to watch your progress as you get stronger and increase your weight. It will also allow you a reference to see what you've done over time, and help you decide when you want to change up your workout.
There are apps that allow you to create your workouts and track your progress while you work out. You'll likely be listening to music on your phone, so this can be a convenient way to track while you go. There's also the tried and tested little notebook, or my favourite (excel nerd alert!), the spreadsheet.
How long you use the same workout is up to you. Over time you will be increasing your weight and adjusting your sets (more on this in the next post), but eventually you'll need, and want, to change up your routine entirely. My suggestion is every few months. This doesn't need to be a radical makeover. It can be a simple as changing out your exercises for one that still hits the same muscle group. This will help beat plateaus by challenging your body to learn a new movement.
Workout Tracker/Journal Apps
If you're looking for something free, or cheap, to get you started, below are a couple of apps I downloaded to my iPhone and gave a quick test run.
Strong (Free) - A basic and straightforward app that lets you create routines by adding your exercises from their library, and tracking weight and reps each time you work out. You can also add your own exercises if they aren't already in the library.
Fitness Point - Gym Buddy (Free Basic, $4.00 to unlock full library) - In addition to creating your own workouts and logging sets and weight, Fitness Point also has information on all exercises in their library. Each exercise has a visual and written description on how to perform it, as well as the muscles it targets.