All visitors to Russia who require a visa need to apply for a visa invitation. Once you have purchased your visa invitation you can apply for your Russian Visa. Please note these are not simple letters from us inviting you, but an official document. Invitations can only be issued by accredited organisations and need to be purchased. Important: you do not have to obtain the invitation from every accommodation and every city you plan to stay. You only need one invitation for your whole stay in Russia.
The visa application form has to be filled in via a website which is common for all embassies. It is advised to collect all necessary information and paperwork (e.g. invitation, travel insurance policy) beforehand, although it is possible to save and continue a form later on. The printed and signed form has to be submitted with a passport photo. Note that there might be some variations regarding to the exact requirements of the application. Some embassies may issue visas by mail, they may require application in person, they may accept a copy of the invitation, they may require the original. They may accept payment by card, they may insist on a money order. Check with the embassy or consulate beforehand - in most cases it will be on their website.
When you arrive to Russia or Belarus you have to go through the passport control. They will check your passport, visa and sometimes additional documents. Then you will receive a small piece of paper called migration card that is very important for your stay in Russia. You might have to fill it out by yourself. Do not lose it! Make a photo in case you lose it. Without this paper you can not check-in into any hotel or hostel.
Registration is required in Russia for every hostel/hotel where you stay. If the accommodation does not register you, they break the law and lead to you breaching the migration regulations which can lead to you be fined and deported from Russia. If you are more than 7 days in Russia and have no registration so far, you have breached the migration law already. So, please, ask for the registration wherever you stay.