Here is a good start for you. This is a detailed tutorial about buying traffic on Udimi https://udimi.com/help/buyer/how-to-buy
There are a few tips for you.
To buy targeted traffic, you can go to the Find Sellers section.
You can search for vendors by niche name and filter them by sales received from their traffic and reported by their buyers.
Carefully check the ratings given by previous traffic buyers to find the right seller for you. Feel free to contact the vendors directly in the Message section if you still have any questions.
Note that in the Solo Deals section, you can find special offers from sellers that can be cheaper than their original price.
There is a lot of useful information in the Help section on the website.
Cheers | John