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.
Anywhere you would selsct an image model (such as in Ads editor's image generation panel, or when selecting tools for a new custom Agent) you can scroll rigth to the bottom of the model selection menu to see any custom LoRAs you may have added. These will be labeled Custom Model by deafault. 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.