I want that my bitcoins automatically were on sale when falling the price. How to make it?

You shall have bitcoin at the crypto exchange and deliver the request for restriction of losses or the so-called stop warrant: in case of achievement of the price the X quantity of bitcoins of N will be exposed in the book of warrants at S price.

If at the crypto exchange execution implementation a market warrants are technically feasible, then you can deliver it. In this case, it is necessary to specify in case of what price of X the quantity of bitcoins of N will be sold. In the latter case, it is not necessary to specify the price at which your request will be exposed in the book of warrants since the market warrant implies momentary sale (in this a case) at the best current price of demand in the market (bid).