Advice on making a raised bed?

Hi! Hopefully this is the right place to ask this question.

I'm currently going through the long process of trying to do up the garden my parents have left unattended for years. We have a concrete patio that goes around the back of the house that would look so nice if it weren't covered in weeds and thorns.

So far we've managed to cut away all the foliage and can actually see the concrete floor. I had an idea that I could build a raised bed at one end of it where I could put perennials/small shrubs in to cover some of it (as the space won't be used anyway).

I've been researching it for the past hour or so, but I'm still unsure about a few things and I would appreciate some help!

• The bed will have a base to it (wooden). It said online that if you do this that you'd need to put a drainage material at the bottom to prevent water-logging.

• This is my question: if I were to say put e.g. sand at the bottom of the soil, if the bed were to get way too much water, would the sand eventually dry out underneath or would it all eventually become waterlogged? If so, would I have to e.g. drill small drainage holes into the wood panels or would water seep between the panels anyway?

• Though this may also be a little vague, apart from drainage and soil quality, is there anything else that I should look out for when making this (e.g. possible mistakes/useful tips for making one etc?).

Thank you in advance!