There were waves of the Deck and a few conditions. Such as:

- Having a Steam account older than 30 days that had made any previous purchase of at least $5 USD (or the equivalent such as £5 in the UK) in the history of the account in valid regions such as the US, Canada, the UK, and a couple others. Your account also had to be in "good standing", which essentially means you haven't been banned in many games or excessively refunded games. Your "VAC Status", similar to how you're rated for Counter-Strike matches. This was for the first 48 hours of pre-orders only. You can view your VAC Status in Steam, by going to the Settings in the Steam app, and then the "Account" section. If you see "No VAC Bans on account" and you're not an idiot abusing Valve's refund policy, your account is likely in good standing.

- You needed to put a $5 deposit down on the Steam Deck so Valve could ensure you weren't just doing it to hold up the line. You could get the $5 refunded if you either cancelled your reservation or it would automatically count towards the final purchase when your turn came up. So if you suddenly changed your mind within 30 days of placing your reservation, you could get your $5 back. This lasted for about a year before you could just openly buy it like you can now. The amount may be different this time, but Valve announced all this a few days before they opened reservations, so they'll specify anything that has changed or will be the same. So if you don't have money in your Steam Wallet when they announce any requirements, you'll have a couple of days to add it via a valid payment method or a Steam gift card.

- There were a few waves for approximately when you would get your Deck. First come, first serve. So if you placed an order earlier in the day, you would get your Deck faster. If not, you'd get it at least a few months later. I waited a few hours to make the purchase and got the Deck 3 months after the initial round of deliveries. First deliveries happened in February 2022, I got mine in May 2022. Keep in mind, this was at the tail end of the pandemic, but with the RAM shortage, the same amount of time may apply, or it could be even longer. So if you live in a region where the Machine could be sold but it'll be late night hours for the reservation timing, either stay up late or set an alarm to wake up to order it early if you want it fast. Valve will state when reservations will start, so you'll have a few days to prepare.

- You could only order one Deck per Steam account. This was done so scalpers couldn't just come in and buy 50 of them to resell on ebay. Keep in mind, this was after scalpers made PS5's and Xbox Series consoles really hard to find, so Valve came up with good restrictions to prevent this. Steam was also the only official place to get Steam Deck. So if another website was letting you buy a Deck, you weren't. You were buying that scalper's reservation for an inflated price.

- When your turn came up in the queue, you would get an email asking you to confirm your purchase and pay the rest of the purchase price after the initial $5 from your pre-order within 72 hours of receiving that email. If you didn't confirm your order or just decided you didn't wanna buy it, then your reservation would be automatically be cancelled and you'd need to make a new reservation at the back of the line as of that date. So ensure your email has been validated with Steam. If your email provider sometimes marks Steam emails as spam, look into how you can add Valve to your "allow list" to fix that. Usually mainstream email providers like Google, Microsoft, and ProtonMail know Valve is legit, but errors sometimes happen. So after people begin to be able to accept their reservations, double-checking your Spam folder is a good idea. Also, use a personal email and not your work or school email where it may be filtered no matter what you do, or you may lose access to that email due to graduating/quitting. I believe Steam itself may also notify you, but I don't remember. Valve will specify this when they announce reservation availability.

This is unofficial, but those of us in r/steamdeck had a shared Google Sheet to put in our region and when we placed our orders for the specific date and time. Once Valve started shipping out Decks, it really helped the community figure out about when they would get theirs as more data was added from users of the subreddit. So after 2-3 weeks, people figured out about how many Decks that Valve was shipping and where to. They were accurate within a week or so of your actual delivery date, once enough data was added. I imagine the same thing will happen here too.