When looking at a players profile there are many options. Near the top left there is a "Create trade" option. Click this to open the trading interface
This is what the trading interface will look like. If only trading one Pokémon, simply left clicking it will add it to the trade. You will know it was selected when above "Your Pokémon" it shows (1/x)
To trade items, click below the Pokémon list where it says "No Items" This will show all the items that you or the trainer you are trading with owns. Choose the item that you want to trade, and enter an amount.
To loan Pokémon, click the text near the bottom of the interface that says "No Pokémon" choose the Pokémon you would like loaned, and type how many hours you would like to loan it for. Keep in mind that you are unable to reclaim the Pokémon until the time is up!