Now that you have your Blood Altar, you will need a way to fill it. To do this you will need a Sacrificial Knife. This is luckily not that hard, and only requires some basic resources.

If you wish, you can also upgrade this Sacrificial Knife to an Arcane Blade, if you have your Infusion Altar set up properly. This will get you Life Essence (LP) more quickly.

The next step in Blood Magic is to make a Blood Orb. This is harder than it sounds however, because it needs a Charged Attuned Stone.
For this you will need to have progressed further in Witchery.

Once you have the Charged Attuned Stone, put it in a Blood Altar which contains some blood. About one health bar in total should do the trick.

The first thing you will want to make with your new Blood Orb is probably a Divination Sigil, seeing as this allows you to check the amount of LP in your Blood Altar.
Something else you shouldn't forget to do is right clicking your Blood Orb, to bind it to yourself.

Now you can get to upgrading your Blood Altar. This will require at least eight Blood Runes, but you should probably make some more. They are also required for some other things.
From here you can keep upgrading your altar, but be aware that all Blood Orb recipes are heavily changed, and require you to progress through ChromatiCraft, Botania, Thaumcraft and Witchery.
Some random hints:
 -Progress through the previously mentioned mods, you will need endgame progress in all of them for the final two orbs.
 -You will need a Bound Sword or an Energy Blaster to get Weak Blood Shards, which are required for the Tier 4 Altar and Master Blood Orb. The recipes for these are altered, and will require you to have unlocked the Thaumcraft Infusion Altar.
 -You may notice that the Ritual Diviner requires a Ritual to make. You can actually build Rituals yourself, so build this one, and then craft a Ritual Diviner to make them in the future.