Greetings!
Step 1: Drive to the front area. If you're coming before 11AM, there will very likely still be a line; park in
Car Park 2 while you're in line, then bring your car to the front area to unload post-checkin - there is a 15 minute limit on parking out the front of the hall.
Step 2: Wait in line (if there is a line when you get there)
Step 3: Be called up to the desk when you reach the front of the line
Step 4: Recite your name, hand over the money, receive your player tag
Step 5: Find a spot to sit, bring in your PC etc.
Step 6: LAAAAAAAAAAN!
--edit--
I have never heard of anyone taking another's place in such a manner. If someone did do this, when you got to the checkin desk and this was noticed, we'd check your ID/confirm you are who you claim you are, then if we found who stole your spot, we'd evict them from the hall and probably perma-ban them.
The person would also not be able to register for competitions etc without knowing your password for the website.