Boosters cannot be sold on contracts. Selling them on the market means market fees. A WTB can be a quick and easy way to let your drug runner of choice fill your order once they can and without everyone having to be online at the same time. Plus, due to the asset window as long as you have an asset at the station you wish to place the WTB contract at, you do not need to be there in person to create it.
It is one of those things that is not hard but it is also not obvious. WTB contracts are very useful for paying someone for something that they are delivering to you. I've tried to teach people who buy boosters from me how to do this but some don't seem to get it. This is not complex. It also bypasses to fees from buy and sell orders, saving money for everyone.
First, just open up your contracts. They can be found under Business - Contracts.
Like every contract ever made, just go down to the create contract button at the bottom. Select 'item exchange' and if you are making it to a particular person, select 'private' and enter their name into the field. If no name is entered, anyone can fill the contract.
This is where WTB contracts shine. As long as you have an item at the station you can write the contract. This allows a lot of remote selling and buying. So just select the station from your list of stations on your item list. Do not select any items at this point. Click next to go to step 3.
Step three is where all the fun is at. There is a click box at the bottom that says 'also request items from the seller'. Click that and a new menu at the bottom of the screen appears. This is where we want to fill out the item that we want to buy. Type it in, type in how many you want and click 'add item'.
Because so many things are named the same Eve will have to ask to make sure you know what you want. This is common with items like boosters where several items are the same with different endings, such as below. Boosters, as far as Eve is concerned, as consumable implants. This is very confusing the first time you see it. But select the right thing.
And poof! Eve now knows what you want. You can continue to add items at this point. Once you have added as many as you want, hit finish.
Now you are just about ready to go. Take a second to read over your details. This is the moment where you get to correct that typo that pays one billion instead of one million.
Now you are ready to spam Jita with scam contracts for PLEX and whatever creative or not so creative things your heart can come up with.