Generally, modding your achievements is risky because you can get perma banned from Xbox Live or even console banned if not done carefully. With that said, to use Horizon’s Achievement Unlocker you can just move your profile to a flash drive then mod it in Horizon like you would anything else.
Unfortunately, games that are only on the Windows Phone cannot be modified with Horizon, only Achievements for games that are also on Xbox can be modified. Also, make sure the Xbox games on your Windows Phone show up on your played games list on your Xbox Achievements list before doing anything with Horizon.
If you have any questions about the process, here’s a thread about Horizon’s profile editor for reference: How do you use account editor. If you have any other questions about this, please post back here and I’ll try and help you out.