Once your LoRA / custom model has been trained you will find it listed at the bottom with the other AI models that you can select.
The screenshot below is taken from the Image Generation screen. Scrolling to the bottom of the model selection menu you will see a LoRA named 'Victorian England' and labeled Custom Model in the top right corner. You can select your custom model just like you would any other model and begin prompting.
See How can I create my own Style LoRA / Custom model for a step by step walkthrough of using a Style LoRA to generate an image.