I got very lost in checking that simplicial sets with Quillen model structure are indeed a simplicial model category. 


Recall that a model category $\mathcal{M}$ is simplicial if it is enriched in $\mathbf{SSet}$, powered by $\mathbf{SSet}$ and tensored by $\mathbf{SSet}$ so that the natural adjunctions exist and and also the enrichment is compatible with the model structure -- namely, for $i: A \to B$ a cofibration in $\mathcal{M}$ and $p: X \to Y$ a fibration in $M$, the map of simplicial sets $i^\*\times p\_\*: \operatorname{Map}(B,X) \to \operatorname{Map}(A,X)\underset{\operatorname{Map}(A,Y)}{\times}\operatorname{Map}(B,Y)$ is required to be a fibration in $\mathbf{SSet}\_{Quillen}$, and it is required to be trivial if either $i$ or $p$ is. 


Recall that for simplicial sets, the enrichment is given by $\operatorname{Map}(K,M)\_n = \mathbf{SSet}(\Delta^n \times K, M).$


$\mathbf{SSet}\_{Quillen}$ is a cofibrantly generated model category, so to check the compatibility one must fill the horns:
$\require{AMScd}$
\begin{CD}
 \Lambda^n\_i @>h>> \operatorname{Map}(B,X) \\
 @V \iota V V @VV i^\* \times p\_\* V\\
 \Delta^n @>(\alpha,\beta)>> \operatorname{Map}(A,X)\underset{\operatorname{Map}(A,Y)}{\times}\operatorname{Map}(B,Y)
\end{CD}


The filling would be the lift in the diagram


$\require{AMScd}$
\begin{CD}
 \Delta^n \times A @>\alpha>> X \\
 @V 1\times i V V @VV p V\\
 \Delta^n \times B@>\beta>> Y
\end{CD}


The natural idea is to consider the diagram


$\require{AMScd}$
\begin{CD}
 \Lambda^n\_i \times B @>h>> X \\
 @V \iota\times 1 V V @VV p V\\
 \Delta^n \times B@>\beta>> Y
\end{CD}


where the left map is a trivial cofibration, the right map is a fibration and thus from the lifting properties I obtain $\sigma: \Delta^n \times B \to X$ satisfying that $p\sigma = \beta$. However, if I want this $\sigma$ to be the answer, I must check $\sigma(1 \times i) = \alpha$... Would that be so? 


I see that I have not made use of $i$ being an inclusion, but I don't see how it helps. 



How are simplicial sets with Quillen model structure a simplicial model category?
The trick is to check that the corner map $$\lambda^n\_k\bar{\times}\delta^m:\Lambda^n\_k \times \Delta^m \coprod\_{\Lambda^n\_k\times \partial \Delta^m} \Delta^n \times \partial \Delta^m \hookrightarrow \Delta^n\times \Delta^m$$ is anodyne for all $k, m, n$ appropriate. This is proven in Higher Topos Theory chapter 2, for your reference.


The fact that $i$ was an inclusion allows you to build it up as a relative cell complex of boundary inclusions, so it is enough to prove the case where $i$ is a boundary inclusion.


