If you have little membership time available
If you have little membership time available, I would strongly suggest increasing magic and melee combat stats. Combat lets you perform more generally, for example skilling in dangerous areas. Magic is very useful for teleports to save huge amounts of traveling time. Agreed with above, but make alot of cash, maybe spend half of your membership time earning money. Money is insanely difficult to earn f2p unless your staking (Which is hard to get kills but you can create millons a hour).
It is really down to personal preference. Magic is the only one that sticks out. All others can be trained much more easily in member areas, anyhow. I waited a long time before becoming a part since I believed my battle level and skills weren't great enough for it to be worth it. Membership really isn't a update just for high levels, though. If you would like to spend less, you could wait a bit, as freeplay still has things to give. However, don't do it if you're going to start looking at the sport for a chore because of it. I would just say that when freeplay is no more fun/as fun, gain membership.
Then cannon any activity which you think is worth it, and for ones you don't (or if you are like me and need battle XP), use piety with superb sets and prayer potions, or whatever battle prayers you can use (that is why you use neitiznot/proselyte legs for the +prayer).
A regular slayer helm is Face Mask, Nose Peg, Spiny Helm, and Black Mask like arham said. To make it a complete slayer helm you'll need to bring a Focus Sight along with a Hexcrest for your slayer helm. For your master, a general rule for Slayer would be to simply use your best master, however in case you want the fastest points you can do 9 jobs with Mazchena (sp?) , the demon man in Canifis, and then do your 10th tasks with your highest available master (Sumona+).
I will buy the berserker ring and prayer fostering armor. I am only able to use the 15% assault and strength bonus though, not yet chivalry or piety. Super potions are the reccomended thing afterward, right? I want to train strength, which means you have all been saying d scim, SS, or korassi's (that I do not now have). I have got the RFD gloves, but maybe not yet nez. Is black mask okay until complete slayer helm? I'll use earmuffs along with other thoughts things instead until then too. Is the prossy hauberk better than fighter torso?
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