At the Penas Blancas border crossing try to get to the border crossing as early as possible to avoid long queues. On entering Nicaragua, there will be people offering to help you with your paperwork or carry your luggage for a small tip, just decline politely but firmly.
You will have to complete an immigration form, have your passport stamped and pay US$10 for a tourist card, US$2 for immigration and US$1 for the local municipality, all at different windows of the immigration office. Motorists have to pay an extra US$6 per person for insurance. Make sure you get all the correct stamps and keep all the receipts, as you will need to produce them when you leave. There is a US$3 charge on leaving. There are toilet facilities on both sides of the border that charge a nominal sum but accept both colones and córdoba.
Please note that Nicaragua, Guatemala, El Salvador and Honduras are part of the Central America Border Control Agreement (CA-4), which means that once you've entered one country, you can travel to/from the others for up to 90 days without additional permits or immigration checks at borders.