I used them recently, having not got on with crypto previously.
Register on coinbase ( some reason took 4 months to authorise my account ?)
Then it allows you to buy which ever crypto you want and its kept on coinbase and you see what you have and how much it cost you. Obviously crypto goes up and down but if you are buying bitcoin and then sending it within 5 or 10 minutes it wont have gone up or down any significant amount.
There is a button to click if you want to buy something and you then input the huge number the seller provides and the amount in money you want to send.
Thats it. Coinbase convert the cash amount into how much of your crypto it needs to cover that amount.
Ie. You buy £200 of bitcoin, it becomes bitcoin. You buy something for £150 in bitcoin it leaves you however much bitcoin you have left. If you bought £200 in bitcoin and it increased in value to £250 then you spend £150 you have £100 in bitcoin left. ( minus the fees of 3%)
Once you have your coinbase account set up its easy, even for an old fucker like me.