Hi everyone. This is my first post. I've been reading this site as well as a FB group for Capital One Venture X for months now and thought I understood how to transfer miles to book a flight, but I actually tried it this morning and it turns out I know very little because the process did not go smoothly and I eventually gave up. I'm hoping you will offer me some guidance because I clearly am not understanding and I don't want to mess up. I'm looking at flying to IAD to London and Amsterdam to IAD in December.

I got on Google Flights and found the flights I wanted with United. I already have an account with United which would cover about 500$ of the flights. I went to the United site to book and was able to choose my seats, although for some reason it showed all the business/premium seats as unavailable, so I chose basic economy plus or something like that, the ones with no one in front of you.

Next I went to to the Cap1 portal and was going to transfer my points, but wasn't sure I was doing it correctly and also wasn't sure how many points I should transfer. When I was on the United site, I couldn't figure out how to choose to pay with all miles, only miles and cash, but maybe that's because I hadn't yet transferred the Cap1 points? At this point, my confidence really dipped, so I tried to book directly on the Cap 1 portal. I would have been able to do that, but couldn't even choose specific economy seats that way. It did show that my 300$ credit would be used, and I guess I could use my points for the rest, but I thought I'd read that it's better to transfer the points to United and book that way. Plus, I normally book directly from the airline in case there's a problem later (particularly since these flights are so far out).

Can someone walk me through the best way to do this step by step or direct me to a thread that breaks it down? I know I've read about it here, on FB and on some of the travel/points websites, but what I remember didn't quite line up with what I've read. I'd also like to understand why I couldn't even see how much premium seats on United cost and why they're not available. I'll be flying overnight, so I might want to spend the extra points for the flight over. Is there a way to know how many United points I'll need for the whole trip and should I use only Cap1 points or also the United points I already have? What else am I not thinking of? I figure I can use the 300$ credit to book lodging.

I sincerely appreciate any help you can offer. I get the feeling this is common sense to most of you, but for some reason I'm just not getting it. Thank you!