What is the right sequence or timing to buy and sell BTC?
The best time to buy Bitcoin or any other cryptocurrency is when the price already went down and hit the support level, but it doesn't always mean that when the price hit the support level that it will not go down anymore. It is impossible to predict the price of cryptocurrencies so you will not always get the right or perfect timing to enter the market.
When BTC drops, then sell all alts to buy BTC, when BTC goes high, then sell BTC for alts?
It shouldn't always work that way and it depends on your preference, if you want to invest in Bitcoin but not good in trading cryptocurrency then why would you risk trading it for alts. Also depends on the situation, if most altcoin price went up together with Bitcoin then it is a dumb move to trade your Bitcoin in altcoins, while the price just recently goes up.
How many percents do you put aside for long-term HODLING?
It depends on you, but I usually hold 60% or more of my Bitcoin and the rest if for trading.