Which champions do you own?
What I did when I was trying to build up my collection was after I got all the core runes I needed, I used my IP to buy all the cheapest champions first--even if I wasn't intending to play them right away. All the ones that are 450ip, then 1350, then 3150 (have like 2 of those left that I don't own), etc. With a champion pool that large, it's easy to find a pick here and there that is flexible enough to use in multiple roles, then just learn the champion really well.
The other thing I did was buy champions that I tried when they were free week when they came on sale, but only if they normally would cost 4800/6300 ip--all the <3150 champs aren't a very good RP value (overpriced compared to buying with ip).
Once you own all the bottom 3 price tiers of champions, you can use the champion shards you get out of crates to randomly roll one you don't own, and you can buy a random champ shard for 1700ip, so if you get at least 1 out of the boxes, you can spend 3400ip for 2 more and roll them for a random one that costs 4800+ (saving you some ip). The only drawback to this method is that you don't get to choose who you get, and it only works if you own all the very cheap champions already.
Bear in mind that this process was over several years. Don't be in a huge hurry to "Collect 'em All!" In the short term, save up for 3 or 4 champions that are flexible and that you enjoy playing. Play them a lot while you build up your champion pool, so you have some comfort picks for those times they don't get picked/banned out and you get the role you want.