Click link to a recent thread very similar to yours.
No special trick needed to put in the self tapping bolts other than to make sure you actually have genuine "self tapping bolts" for the task. Self tapping DOES NOT mean you use a standard bolt and just force it into the new and unthreaded hole in the spindle.
The only other consideration for using self tapping bolts is that you put some pressure on the bolt using a ratchet or pull handle (breaker bar) and a socket to encourage the self tapping bolt to get a proper bite to begin the thread cutting process. It sometimes helps if you remove the bolts after every couple of turns and clean out the chips.
After the self cutting bolt has made some threads you no longer need to apply any downward pressure with your tools, the bolt will pull itself in after it has made some threads in the casting.
IF........the new spindle has a grease zerk in it (or a ready made plugged hole for one) it will do no harm to pump in a little grease. But having said that there is no guarantee that the grease will actually make its way into the bearings. Most of these "throw away" spindles are built using sealed or shielded bearings and if that is the case.....additional grease will not find its way into the bearings anyhow.
Greasing of spindles is something of a bone of contention for many members and there are a few valid caveats associated with that subject.
Bottom line: If it comes with grease fittings......grease them lightly......and often.

Here is a link that might be useful: Sounds familiar

Just installed three of the self tapping bolts in a new spindle, i put a few drops of a 3/1 oil on the bolt, the just drove down with ease.