The proper way to do buy and hold is simply that, you buy and hold it indefinitely, until you need it for it's intended purpose. Both of your ideas are market timing, and unless you're specifically looking for market timing advice, I'd advise against it. There is no way especially for an amateur investor to consistently and accurately predict the tops and bottoms of the market, and the recent trend of up or down is included in that--just because the market goes up, doesn't mean it won't keep going up (this year proves that with the 30% upswing in the S&P 500 index). Same with downturns. If your strategy is buy and hold, then just keep investing regularly throughout the ups and downs.
Dollar (or in your case Euro) cost averaging is the general approach I and many others would recommend--for every savings period, add the same amount in Euros to your mix of mutual funds/ETFs/etc. This does naturally build in a bit of buy low/sell high, because the same Euros will buy more shares when they are cheaper, and less when they are more expensive.
I have money automatically invested for me out of each paycheck, every 2 weeks, to my retirement account and my health savings account. Quarterly seems like a pretty long time to wait between investments to me, but if the amounts are small enough and you don't have a zero or low transaction cost option then I suppose it might make sense. If you had a relatively low expense ratio mutual fund option that with more frequent deposits might be my preference, even if once a quarter you transferred it to the ETF/stock or whatever it is you're buying now.