Open the secondary market in your account and choose an offer. The information window shows the remaining amount, the purchase price with all discounts and premiums, and the resulting return. Click "Buy now" to make the purchase.
When buying is worthwhile: If you have already invested in loan projects, the secondary market is a good way to diversify your portfolio further, with running loans and without waiting for new loan projects. Look closely at the offer before you buy: the rating, the remaining term, the instalments paid so far and above all the premium or discount on the purchase price. A premium lowers your return, a discount raises it, see How is the price of a loan made up?
The purchase is binding. You then receive the future instalments of this loan. Borrowers can default, and you can lose capital. This applies in exactly the same way to loans bought on the secondary market.